don't fail pthread_sigmask/sigprocmask on invalid how when set is null

the resolution of Austin Group issue #1132 changes the requirement to
fail so that it only applies when the set argument (new mask) is
non-null. this change was made for consistency with the description,
which specified "if set is a null pointer, the value of the argument
how is not significant".

src/thread/pthread_sigmask.c

@@ -5,7 +5,7 @@
 int pthread_sigmask(int how, const sigset_t *restrict set, sigset_t *restrict old)
 {
 	int ret;
-	if ((unsigned)how - SIG_BLOCK > 2U) return EINVAL;
+	if (set && (unsigned)how - SIG_BLOCK > 2U) return EINVAL;
 	ret = -__syscall(SYS_rt_sigprocmask, how, set, old, _NSIG/8);
 	if (!ret && old) {
 		if (sizeof old->__bits[0] == 8) {