Bug#40389: REORGANIZE PARTITION crashes when only using one partition

The non documented command 'ALTER PARTITION t REORGANIZE PARTITION'
(without any partitions!) which only make sense for nativly
partitioned engines, such as NDB, crashes the server if there was
no change of number of partitions.

The problem was wrong usage of fast_end_partition function,
which led to usage of a non initialized variable.

mysql-test/r/partition_mgm.result

 DROP TABLE IF EXISTS t1;
+CREATE TABLE t1 (a INT PRIMARY KEY)
+ENGINE MYISAM
+PARTITION BY HASH (a)
+PARTITIONS 1;
+INSERT INTO t1 VALUES (1),(2),(3),(4),(5);
+ALTER TABLE t1 REORGANIZE PARTITION;
+DROP TABLE t1;
 create table t1 (a int)
 partition by range (a)
 subpartition by key (a)

mysql-test/t/partition_mgm.test

@@ -3,6 +3,17 @@
 DROP TABLE IF EXISTS t1;
 --enable_warnings
 
+#
+# Bug 40389: REORGANIZE PARTITION crashes when only using one partition
+#
+CREATE TABLE t1 (a INT PRIMARY KEY)
+ENGINE MYISAM
+PARTITION BY HASH (a)
+PARTITIONS 1;
+INSERT INTO t1 VALUES (1),(2),(3),(4),(5);
+ALTER TABLE t1 REORGANIZE PARTITION;
+DROP TABLE t1;
+
 #
 # Bug 21143: mysqld hang when error in number of subparts in
 #            REORGANIZE command

sql/partition_info.cc

@@ -867,6 +867,7 @@ bool partition_info::check_partition_info(THD *thd, handlerton **eng_type,
 
     if (part_type != HASH_PARTITION || !list_of_part_fields)
     {
+      DBUG_ASSERT(part_expr);
       err= part_expr->walk(&Item::check_partition_func_processor, 0,
                            NULL);
       if (!err && is_sub_partitioned() && !list_of_subpart_fields)

sql/sql_partition.cc

@@ -4228,9 +4228,8 @@ uint prep_alter_part_table(THD *thd, TABLE *table, Alter_info *alter_info,
           after the change as before. Thus we can reply ok immediately
           without any changes at all.
         */
-        DBUG_RETURN(fast_end_partition(thd, ULL(0), ULL(0),
-                                       table, NULL,
-                                       TRUE, NULL, FALSE));
+        *fast_alter_partition= TRUE;
+        DBUG_RETURN(FALSE);
       }
       else if (new_part_no > curr_part_no)
       {