Set the port in fake SDP answer (to the other side)
coming from the leg, which now replaces the previously
existing one, to the port gotten from the replace INVITE.
Otherwise, if to use the port from the pending transaction
(towards pick-up'er) it confuses rtpengine with a case
when same port talks to himself, and then can later be
used to update the pick-uper's leg once again.
Change-Id: I4c001f7956a005b64285c05586839f01216cfd91