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.

(cherry picked from d8abf71ed74a739749ab34e0e823ac2fe9c90200)
spiral: Fix CANCEL behaviour (and CANCEL test).