fix crash when signal number 0 is passed to sigaction

this error case was overlooked in the old range checking logic. new
check is moved out of __libc_sigaction to the public wrapper in order
to unify the error path and reduce code size.

src/signal/sigaction.c

@@ -17,10 +17,6 @@ void __get_handler_set(sigset_t *set)
 int __libc_sigaction(int sig, const struct sigaction *restrict sa, struct sigaction *restrict old)
 {
 	struct k_sigaction ksa, ksa_old;
-	if (sig >= (unsigned)_NSIG) {
-		errno = EINVAL;
-		return -1;
-	}
 	if (sa) {
 		if ((uintptr_t)sa->sa_handler > 1UL) {
 			a_or_l(handler_set+(sig-1)/(8*sizeof(long)),
@@ -57,7 +53,7 @@ int __libc_sigaction(int sig, const struct sigaction *restrict sa, struct sigact
 
 int __sigaction(int sig, const struct sigaction *restrict sa, struct sigaction *restrict old)
 {
-	if (sig-32U < 3) {
+	if (sig-32U < 3 || sig-1U >= _NSIG-1) {
 		errno = EINVAL;
 		return -1;
 	}