Fixed bug #25027.

Removed an assertion that was not valid for the cases where the query
in a prepared statement contained a single-row non-correlated
subquery that was used as an argument of the IS NULL predicate.

mysql-test/r/ps.result

@@ -1514,4 +1514,19 @@ Variable_name	Value
 Slow_queries	1
 deallocate prepare no_index;
 deallocate prepare sq;
+CREATE TABLE t1 (a int);
+INSERT INTO t1 VALUES (1), (2);
+CREATE TABLE t2 (b int);
+INSERT INTO t2 VALUES (NULL);
+SELECT a FROM t1 WHERE (SELECT b FROM t2) IS NULL;
+a
+1
+2
+PREPARE stmt FROM 'SELECT a FROM t1 WHERE (SELECT b FROM t2) IS NULL';
+EXECUTE stmt;
+a
+1
+2
+DEALLOCATE PREPARE stmt;
+DROP TABLE t1,t2;
 End of 5.0 tests.

mysql-test/t/ps.test

@@ -1563,4 +1563,22 @@ execute sq;
 deallocate prepare no_index;
 deallocate prepare sq;
 
+#
+# Bug 25027: query with a single-row non-correlated subquery
+#            and IS NULL predicate
+#
+
+CREATE TABLE t1 (a int);
+INSERT INTO t1 VALUES (1), (2);
+CREATE TABLE t2 (b int);
+INSERT INTO t2 VALUES (NULL);
+
+SELECT a FROM t1 WHERE (SELECT b FROM t2) IS NULL;
+PREPARE stmt FROM 'SELECT a FROM t1 WHERE (SELECT b FROM t2) IS NULL';
+
+EXECUTE stmt;
+
+DEALLOCATE PREPARE stmt;
+DROP TABLE t1,t2;
+
 --echo End of 5.0 tests.

sql/sql_lex.cc

@@ -1903,7 +1903,6 @@ void st_select_lex_unit::set_limit(SELECT_LEX *sl)
 {
   ha_rows select_limit_val;
 
-  DBUG_ASSERT(! thd->stmt_arena->is_stmt_prepare());
   select_limit_val= (ha_rows)(sl->select_limit ? sl->select_limit->val_uint() :
                                                  HA_POS_ERROR);
   offset_limit_cnt= (ha_rows)(sl->offset_limit ? sl->offset_limit->val_uint() :