Fixed LP bug #972973.

When the function free_tmp_table deletes the handler object for
a temporary table the field TABLE::file for this table should be
set to NULL. Otherwise an assertion failure may occur.

sql/sql_select.cc

@@ -15116,6 +15116,7 @@ free_tmp_table(THD *thd, TABLE *entry)
     else
       entry->file->ha_delete_table(entry->s->table_name.str);
     delete entry->file;
+    entry->file= 0;
   }
 
   /* free blobs */