msg_translator: Passing MSG_TRANS_NOVIA_FLAG means don't touch Via.

Before this, if you passed the MSG_TRANS_NOVIA_FLAG flag to
build_res_buf_from_sip_res, the msg->via1->next would get checked but
not used. Now it's not touched at all.
Merge pull request #305 from wdoekes/wjd-msg_trans_no_via-no_crash

msg_translator: Passing MSG_TRANS_NOVIA_FLAG means don't touch Via.