net/socket: fix coverity issue

This fixes coverity issue CID1005339.

Make sure that saddr is not used uninitialized if the
mcast parameter is NULL.

Cc: qemu-stable@nongnu.org
Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Jens Freimann <jfreimann@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>