Fix for bug #13825 "Triggers: crash if release savepoint" and for general

handling of savepoints in stored routines.

Fixed ha_rollback_to_savepoint()/ha_savepoint()/ha_release_savepoint()
functions to properly handle savepoints inside of stored functions and
triggers.
Also now when we invoke stored function or trigger we create new savepoint
level. We destroy it at the end of function/trigger execution and return back
to old savepoint level.

sql/sql_class.h

@@ -1097,6 +1097,7 @@ public:
   uint in_sub_stmt;
   bool enable_slow_log, insert_id_used;
   my_bool no_send_ok;
+  SAVEPOINT *savepoints;
 };