Fix crash in TCPSocket.open

Fix segfault crash observable with TCPSocket.open(nil, nil)

Author: lukejahnke

ext/socket/ipsocket.c

@@ -593,7 +593,7 @@ init_fast_fallback_inetsock_internal(VALUE v)
         arg->getaddrinfo_shared->notify = hostname_resolution_notifier;
 
         arg->getaddrinfo_shared->node = arg->hostp ? ruby_strdup(arg->hostp) : NULL;
-        arg->getaddrinfo_shared->service = ruby_strdup(arg->portp);
+        arg->getaddrinfo_shared->service = arg->portp ? ruby_strdup(arg->portp) : NULL;
         arg->getaddrinfo_shared->refcount = arg->family_size + 1;
 
         for (int i = 0; i < arg->family_size; i++) {