Bounds check the argument to FD_SET

Failing to check the FD_SET argument against FD_SETSIZE causes
undefined behaviour (segfaults, for example).

src/usocket.c

@@ -49,6 +49,7 @@ int socket_waitfd(p_socket ps, int sw, p_timeout tm) {
     fd_set rfds, wfds, *rp, *wp;
     struct timeval tv, *tp;
     double t;
+    if (*ps >= FD_SETSIZE) return EINVAL;
     if (timeout_iszero(tm)) return IO_TIMEOUT;  /* optimize timeout == 0 case */
     do {
         /* must set bits within loop, because select may have modifed them */