Ensure thread library is initialized in pthread_testcancel().

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.

Reported by:	arichardson
Reviewed by:	kib
Obtained from:	CheriBSD
MFC after:	1 week
Sponsored by:	DARPA / AFRL
Differential Revision:	https://reviews.freebsd.org/D14786