slirp: fork_exec(): Don't close() a negative number in fork_exec()

In a fork_exec() error path we try to closesocket(s) when s might
be a negative number because the thing that failed was the
qemu_socket() call. Add a guard so we don't do this.

(Spotted by Coverity: CID 1005727 issue 1 of 2.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
