Fixed rare core dump bug when using complicated GROUP BY query that didn't return any results.

This only showed up under heavy load.

sql/sql_select.cc

@@ -4970,6 +4970,8 @@ end_write_group(JOIN *join, JOIN_TAB *join_tab __attribute__((unused)),
     }
     else
     {
+      if (end_of_records)
+	DBUG_RETURN(0);
       join->first_record=1;
       VOID(test_if_group_changed(join->group_fields));
     }