Ensure that threading library is initialized in pthread_mutex_init().

We need at least thr_malloc ready.  The situation is possible e.g. in case
of libthr being listed in DT_NEEDED before some of its consumers.

Reported and tested by:	lev
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week

lib/libthr/thread/thr_mutex.c

@@ -384,6 +384,8 @@ __Tthr_mutex_init(pthread_mutex_t * __restrict mutex,
 	struct pthread_mutex *pmtx;
 	int ret;
 
+	_thr_check_init();
+
 	if (mutex_attr != NULL) {
 		ret = mutex_check_attr(*mutex_attr);
 		if (ret != 0)