Dropping an user defined function may cause server crash in
case this function is still in use by another thread.

The problem was that our hash implementation didn't update
hash link list properly when hash_update() was called.


mysys/hash.c:
  The following requirement wasn't met by hash_update() function
  causing corruption of hash links list:
  
  After a record was unlinked from the old chain during update, it
  holds random position. By the chance this position is equal to
  position for the first element in the new chain. That means
  updated record is the only record in the new chain.