Bug#38231: Innodb crash in lock_reset_all_on_table() on TRUNCATE + LOCK / UNLOCK

branches/5.1:

Fix Bug#38231 Innodb crash in lock_reset_all_on_table() on TRUNCATE + LOCK / UNLOCK

In TRUNCATE TABLE and discard tablespace: do not remove table-level S
and X locks and do not assert on such locks not being wait locks.
Leave such locks alone.

Approved by:    Heikki (rb://14)