qemu-socket: catch monitor_get_fd failures

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

util/qemu-sockets.c

@@ -903,7 +903,7 @@ int socket_connect(SocketAddress *addr, Error **errp,
 
     case SOCKET_ADDRESS_KIND_FD:
         fd = monitor_get_fd(cur_mon, addr->fd->str, errp);
-        if (callback) {
+        if (fd >= 0 && callback) {
             qemu_set_nonblock(fd);
             callback(fd, opaque);
         }