BUG#20919 temp tables closing fails when binlog is off

closing temp tables through end_thread
had a flaw in binlog-off branch of close_temporary_tables where
next table to close was reset via table->next
 for (table= thd->temporary_tables; table; table= table->next)
which was wrong since the current table instance got destoyed at
	close_temporary(table, 1);

The fix adapts binlog-on branch method to engage the loop's internal 'next' variable which holds table->next prior table's destoying.

sql/sql_base.cc

@@ -499,8 +499,10 @@ void close_temporary_tables(THD *thd)
 
   if (!mysql_bin_log.is_open())
   {
-    for (table= thd->temporary_tables; table; table= table->next)
+    TABLE *next;
+    for (table= thd->temporary_tables; table; table= next)
     {
+      next= table->next;
       close_temporary(table, 1);
     }
     thd->temporary_tables= 0;