Fix undefined behavior in php_set_inet6_addr

Postfix `++` on `NULL` is undefined behavior

Closes GH-8607

Author: iluuu1994

NEWS

@@ -110,6 +110,7 @@ PHP                                                                        NEWS
 
 - Sockets:
   . Fixed Solaris builds. (David Carlier)
+  . Fix undefined behavior in php_set_inet6_addr. (ilutov)
 
 - SPL:
   . Fixed bug GH-8121 (SplFileObject - seek and key with csv file inconsistent).

ext/sockets/sockaddr_conv.c

@@ -60,11 +60,13 @@ int php_set_inet6_addr(struct sockaddr_in6 *sin6, char *string, php_socket *php_
 
 	}
 
-	if (scope++) {
+	if (scope) {
 		zend_long lval = 0;
 		double dval = 0;
 		unsigned scope_id = 0;
 
+		scope++;
+
 		if (IS_LONG == is_numeric_string(scope, strlen(scope), &lval, &dval, 0)) {
 			if (lval > 0 && (zend_ulong)lval <= UINT_MAX) {
 				scope_id = lval;