Fixed autobridging in combination with TopoHiding.

Get information about the next hop from the Transaction layer (when Topo Hiding is used), as we have no second VIA present in the SIP replies.
Based on an original fix from Nick Altmann, related to push request #238.

(cherry picked from commit 20d1fe67de5c70ff1ff8a56912c515f1b88ae6bb)