slirp: Check qemu_socket() return value in udp_listen()

Check the return value from qemu_socket() rather than trying to
pass it to bind() as an fd argument even if it's negative.
This wouldn't have caused any negative consequences, because
it won't be a valid fd number and the bind call will fail;
but Coverity complains (CID 1005723).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
