Fix fsck_ufs segfault when it needs to rerun.

The segfault was being hit in the rerun of Pass 1 in ginode() when
trying to get an inode that needs to be repaired. When the first run
of fsck_ffs finishes it clears the inode cache, but ginode() was
failing to check properly and tried to access the deallocated cache entry.

Reported by: Peter Holm
Reviewed by: Chuck Silvers
Tested by: Peter Holm and Chuck Silvers
MFC after: 3 days
Sponsored by: Netflix