Don't add sockfd to fd_set if it is invalid.

serial_poll() only uses the validity of the connfd descriptor to
determine if the serial module has been initialized, but it must also
check if the sockfd has been opened before accepting connections on it.

Reported-by: Luigi Leonardi <[email protected]>

Author: giomba

src/serial.c

@@ -48,6 +48,9 @@ static void serial_poll(void) {
     timeout.tv_sec = 0;
     timeout.tv_usec = 0;
 
+    if (sockfd < 0)
+        return;
+
     if (connfd == -1) {
         fd_set accept_set;
         FD_ZERO(&accept_set);