MFC r294565: sem: Don't free nameinfo that is still in list when open()

MFC r294565: sem: Don't free nameinfo that is still in list when open()
fails.

This bug could be reproduced easily by calling sem_open() with O_CREAT |
O_EXCL on a semaphore that is already open in the process. The struct
sem_nameinfo would be freed while still in sem_list and later calls to
sem_open() or sem_close() could access freed memory.

PR:		206396

git-svn-id: svn://svn.freebsd.org/base/stable/10@294963 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f