MDEV-35611 ALTER IF EXISTS assertions in sql_errno with statement timeout

Check stmt_da::is_error before calling stmt_da::sql_errno(which is what
the assertion ensures)

The bug did not have any negative effects in optimized builds.

Author: vaintroub

mysql-test/main/alter_table_lock.result

@@ -60,3 +60,22 @@ drop table t1;
 #
 # End of 10.11 tests
 #
+#
+# MDEV-35611 Assertion failure in Diagnostics_area::sql_errno upon interrupted ALTER
+#
+CREATE TABLE t (a INT) ENGINE=MyISAM;
+INSERT INTO t VALUES (1);
+LOCK TABLE t READ;
+connection con1;
+SET max_statement_time=0.001;
+ALTER TABLE t FORCE;
+ERROR 70100: Query execution was interrupted (max_statement_time exceeded)
+ALTER TABLE IF EXISTS t FORCE;
+ERROR 70100: Query execution was interrupted (max_statement_time exceeded)
+disconnect con1;
+connection default;
+UNLOCK TABLES;
+DROP TABLE t;
+#
+# End of 11.4 tests
+#

mysql-test/main/alter_table_lock.test

@@ -78,3 +78,25 @@ drop table t1;
 --echo #
 --echo # End of 10.11 tests
 --echo #
+
+--echo #
+--echo # MDEV-35611 Assertion failure in Diagnostics_area::sql_errno upon interrupted ALTER
+--echo #
+CREATE TABLE t (a INT) ENGINE=MyISAM;
+INSERT INTO t VALUES (1);
+LOCK TABLE t READ;
+--connection con1
+SET max_statement_time=0.001;
+--error ER_STATEMENT_TIMEOUT
+ALTER TABLE t FORCE;
+--error ER_STATEMENT_TIMEOUT
+ALTER TABLE IF EXISTS t FORCE;
+# Cleanup
+--disconnect con1
+--connection default
+UNLOCK TABLES;
+DROP TABLE t;
+
+--echo #
+--echo # End of 11.4 tests
+--echo #

sql/sql_table.cc

@@ -10655,7 +10655,7 @@ bool mysql_alter_table(THD *thd, const LEX_CSTRING *new_db,
 
   if (unlikely(error))
   {
-    if (if_exists)
+    if (if_exists && thd->get_stmt_da()->is_error())
     {
       int tmp_errno= thd->get_stmt_da()->sql_errno();
       if (tmp_errno == ER_NO_SUCH_TABLE)