Bug#35904044: failure in Protocol_classic::send_field_metadata

There may be a failure when returning metadata to the client for certain
SQL queries involving dynamic parameters and subqueries in a SELECT
clause. The fix is to avoid setting an item name that is a NULL pointer.

Change-Id: I1abe206f97060c218de1ae23c63a4da80ffaaae5

Author: roylyseng

sql/item_subselect.cc

@@ -560,7 +560,9 @@ bool Item_subselect::fix_fields(THD *thd, Item **ref) {
     if (substitution) {
       int ret = 0;
       (*ref) = substitution;
-      substitution->item_name = item_name;
+      if (item_name.is_set()) {
+        substitution->item_name = item_name;
+      }
       if (have_to_be_excluded) {
         unit->exclude_level();
       }