if two threads were calling mi_assign_to_key_cache() for the same table,
one could change share->key_cache while the other was having
share->key_cache->op_lock locked. The other thread would crash then,
trying to unlock share->key_cache->op_lock (because it would be a different mutex).

fixed by caching the value of share->key_cache in a local variable. The thread can still
call flush_key_blocks() for an unassigned keycache, but it's harmless.