slirp: don't zero the whole ti_i when m == NULL

98c63057 ('slirp: Factorizing
tcpiphdr structure with an union') introduced a memset call to clear
possibly-undefined fields in ti. This however overwrites src/dst/pr which
are used below.

So let us clear only the unused fields.

This should fix some rare cases (some RST cases, keep alive probes)
where packets would be sent to 0.0.0.0.

Signed-off-by: Tao Wu <lepton@google.com>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>