slirp: Don't pass possibly -1 fd to send()

slirp: Don't pass possibly -1 fd to send()

Coverity complains (CID 1005726) that we might pass -1 as the fd
argument to send() in slirp_send(), because we previously checked for
"so->s == -1 && so->extra".  The case of "so->s == -1 but so->extra
NULL" should not in theory happen, but it is hard to guarantee
because various places in the code do so->s = qemu_socket(...) and so
will end up with so->s == -1 on failure, and not all the paths which
call that always throw away the socket in that case (eg
tcp_fconnect()).  So just check specifically for the condition and
fail slirp_send().

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>