MFC 331324: Ensure thread library is initialized in pthread_testcancel().

MFC 331324: Ensure thread library is initialized in pthread_testcancel().

Call _thr_check_init() before reading curthread in pthread_testcancel().

If a constructor in a library creates a semaphore via sem_init() and
then waits for it via sem_wait(), the program can core dump in
_pthread_testcancel() called from sem_wait().  This is because the
semaphore implementation lives in libc, so the library's constructors
can be run before libthr's constructors.

Sponsored by:	DARPA / AFRL

git-svn-id: svn://svn.freebsd.org/base/stable/10@332633 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f