Spider calls ha_spider::close() at least twice on ALTER TABLE ... ADD
PARTITION. The first call frees wide_handler and the second call
accesses wide_handler->trx->thd (heap-use-after-free).

In general, there seems to be no problem with using THD obtained by
the macro current_thd() except in background threads. Thus, we simply
replace wide_handler->trx->thd with current_thd().

Original author: Nayuta Yanagasawa