sem_remove(): add some asserts.

Assert that sema[idx] allocation from sem[] is sane.
Also assert that sem_mtx is owned, it protects the SEM_ALLOC flag.

Reviewed by: markj
Tested by: pho
Sponsored by: The FreeBSD Foundation (kib)
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D23694