MDEV-19452 - fix incorrect push_warning_printf

String is not guaranteed to be null-terminated, thus push_warning_printf
also needs to specify the length.

sql/item_subselect.cc

@@ -711,11 +711,14 @@ bool Item_subselect::exec()
   DBUG_ASSERT(fixed);
 
   DBUG_EXECUTE_IF("Item_subselect",
-                  push_warning_printf(thd, Sql_condition::WARN_LEVEL_NOTE,
-                  ER_UNKNOWN_ERROR, "DBUG: Item_subselect::exec %s",
-                  Item::Print(this,
-                              enum_query_type(QT_TO_SYSTEM_CHARSET |
-                                              QT_WITHOUT_INTRODUCERS)).ptr()););
+    Item::Print print(this,
+      enum_query_type(QT_TO_SYSTEM_CHARSET |
+        QT_WITHOUT_INTRODUCERS));
+
+    push_warning_printf(thd, Sql_condition::WARN_LEVEL_NOTE,
+       ER_UNKNOWN_ERROR, "DBUG: Item_subselect::exec %.*s",
+       print.length(),print.ptr());
+  );
   /*
     Do not execute subselect in case of a fatal error
     or if the query has been killed.