Fixed LP bug #794005.

The function st_table::mark_virtual_columns_for_write() did not take into
account the fact that for any table the value of st_table::vfield is 0
when there are no virtual columns in the table definition.

mysql-test/r/view.result

@@ -3926,3 +3926,14 @@ drop table t1,t2;
 # -----------------------------------------------------------------
 # -- End of 5.1 tests.
 # -----------------------------------------------------------------
+#
+# Bug #794005: crash in st_table::mark_virtual_columns_for_write
+#
+CREATE TABLE t1 (a int);
+CREATE TABLE t2 (a int);
+CREATE VIEW v2 AS SELECT * FROM t2;
+CREATE VIEW v1 AS SELECT * FROM v2;
+CREATE OR REPLACE ALGORITHM = TEMPTABLE VIEW v2 AS SELECT * FROM t1;
+UPDATE v1 SET a = 10;
+DROP VIEW v1,v2;
+DROP TABLE t1,t2;

mysql-test/t/view.test

@@ -3976,3 +3976,19 @@ drop table t1,t2;
 --echo # -----------------------------------------------------------------
 --echo # -- End of 5.1 tests.
 --echo # -----------------------------------------------------------------
+
+--echo #
+--echo # Bug #794005: crash in st_table::mark_virtual_columns_for_write
+--echo #
+
+CREATE TABLE t1 (a int);
+CREATE TABLE t2 (a int);
+
+CREATE VIEW v2 AS SELECT * FROM t2;
+CREATE VIEW v1 AS SELECT * FROM v2;
+CREATE OR REPLACE ALGORITHM = TEMPTABLE VIEW v2 AS SELECT * FROM t1;
+
+UPDATE v1 SET a = 10;
+
+DROP VIEW v1,v2;
+DROP TABLE t1,t2;

sql/table.cc

@@ -5161,6 +5161,9 @@ void st_table::mark_virtual_columns_for_write(bool insert_fl)
   Field **vfield_ptr, *tmp_vfield;
   bool bitmap_updated= FALSE;
 
+  if (!vfield)
+    return;
+
   for (vfield_ptr= vfield; *vfield_ptr; vfield_ptr++)
   {
     tmp_vfield= *vfield_ptr;