Fix destruction of the robust mutexes.

If robust mutex' owner terminated, causing kernel-assisted state
recovery, and then pthread_mutex_destroy() is executed as the next
action, assert is triggered about mutex still being on the list.
Ignore the mutex linkage in pthread_mutex_destroy() for shared robust
mutexes with dead owner, same as for enqueue_mutex().

Reported by: avg
Sponsored by: The FreeBSD Foundation
MFC after: 1 week