fix error-path NULL deref in alloc_posix_timer()

Found by static checker (http://repo.or.cz/w/smatch.git).
Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/posix-timers.c

@@ -441,7 +441,7 @@ static struct k_itimer * alloc_posix_timer(void)
 		return tmr;
 	if (unlikely(!(tmr->sigq = sigqueue_alloc()))) {
 		kmem_cache_free(posix_timers_cache, tmr);
-		tmr = NULL;
+		return NULL;
 	}
 	memset(&tmr->sigq->info, 0, sizeof(siginfo_t));
 	return tmr;