MDEV-5217: Unlock of de-allocated mutex

There was a race in the code for wait_for_commit::wakeup().

Since the waiter does a dirty read of the waiting_for_commit
flag, it was possible for the waiter to complete and deallocate
the wait_for_commit object while the waitee was still running
inside wakeup(). This would cause the waitee to access invalid
memory.

Fixed by putting an extra lock/unlock in the destructor for
wait_for_commit, to ensure that waitee has finished with the
object before it is deallocated.