MDEV-11737 Failing assertion: block->magic_n == MEM_BLOCK_MAGIC_N

Issue was that the m_prebuilt array was reused without resetting a counter,
which caused a memory overrun.

Adjusted test case to 79 characters

mysql-test/suite/vcol/r/vcol_keys_innodb.result

@@ -215,3 +215,13 @@ INDEX(vc(32))
 );
 DELETE FROM t1 WHERE vc IS NULL ORDER BY pk;
 DROP TABLE t1;
+#
+# MDEV-11737 Failing assertion: block->magic_n == MEM_BLOCK_MAGIC_N
+# 
+CREATE TABLE t1 (i INT PRIMARY KEY, vi INT AS (i*2) VIRTUAL UNIQUE)
+ENGINE=InnoDB;
+CREATE TABLE t2 (i INT) ENGINE=InnoDB;
+ALTER TABLE t1 ADD COLUMN col INT;
+SELECT * FROM t1 WHERE vi < 2;
+i	vi	col
+DROP TABLE t1, t2;

mysql-test/suite/vcol/t/vcol_keys_innodb.test

-################################################################################
-# t/vcol_keys_innodb.test                                                      #
-#                                                                              #
-# Purpose:                                                                     #
-#  Testing keys, indexes defined upon virtual columns.                         # 
-#                                                                              #
-#        InnoDB branch                                                         #
-#                                                                              #
-#------------------------------------------------------------------------------#
-# Original Author: Andrey Zhakov                                               #
-# Original Date: 2008-09-04                                                    #
-# Change Author:                                                               #
-# Change Date:                                                                 #
-# Change:                                                                      #
-################################################################################
+###############################################################################
+# t/vcol_keys_innodb.test                                                     #
+#                                                                             #
+# Purpose:                                                                    #
+#  Testing keys, indexes defined upon virtual columns.                        #
+#                                                                             #
+#        InnoDB branch                                                        #
+#                                                                             #
+#-----------------------------------------------------------------------------#
+# Original Author: Andrey Zhakov                                              #
+# Original Date: 2008-09-04                                                   #
+# Change Author:                                                              #
+# Change Date:                                                                #
+# Change:                                                                     #
+###############################################################################
 
 #
-# NOTE: PLEASE DO NOT ADD NOT MYISAM SPECIFIC TESTCASES HERE !
+# NOTE: PLEASE DO NOT ADD NOT INNODB SPECIFIC TESTCASES HERE !
 #       TESTCASES WHICH MUST BE APPLIED TO ALL STORAGE ENGINES MUST BE ADDED IN
 #       THE SOURCED FILES ONLY.
 #
 
-#------------------------------------------------------------------------------#
+#-----------------------------------------------------------------------------#
 # General not engine specific settings and requirements
 --source suite/vcol/inc/vcol_init_vars.pre
 
-#------------------------------------------------------------------------------#
+#-----------------------------------------------------------------------------#
 # Cleanup
 --source suite/vcol/inc/vcol_cleanup.inc
 
-#------------------------------------------------------------------------------#
+#-----------------------------------------------------------------------------#
 # Engine specific settings and requirements
 
 ##### Storage engine to be tested
@@ -39,14 +39,25 @@ SET @@session.storage_engine = 'InnoDB';
 ##### Workarounds for known open engine specific bugs
 #     none
 
-#------------------------------------------------------------------------------#
+#-----------------------------------------------------------------------------#
 # Execute the tests to be applied to all storage engines
 let $with_foreign_keys = 1;
 --source suite/vcol/inc/vcol_keys.inc
 
-#------------------------------------------------------------------------------#
-# Execute storage engine specific tests
-
-#------------------------------------------------------------------------------#
 # Cleanup
 --source suite/vcol/inc/vcol_cleanup.inc
+
+#-----------------------------------------------------------------------------#
+# Execute storage engine specific tests
+#-----------------------------------------------------------------------------#
+
+--echo #
+--echo # MDEV-11737 Failing assertion: block->magic_n == MEM_BLOCK_MAGIC_N
+--echo # 
+
+CREATE TABLE t1 (i INT PRIMARY KEY, vi INT AS (i*2) VIRTUAL UNIQUE)
+ENGINE=InnoDB;
+CREATE TABLE t2 (i INT) ENGINE=InnoDB;
+ALTER TABLE t1 ADD COLUMN col INT;
+SELECT * FROM t1 WHERE vi < 2;
+DROP TABLE t1, t2;

storage/innobase/handler/ha_innodb.cc

@@ -8497,7 +8497,10 @@ ha_innobase::build_template(
 				index_contains = dict_index_contains_col_or_prefix(
 					index, num_v, true);
                                 if (index_contains)
+                                {
+                                        m_prebuilt->n_template = 0;
                                         goto no_icp;
+                                }
 			} else {
 				index_contains = dict_index_contains_col_or_prefix(
 					index, i - num_v, false);