Description of the bug:
When a dialog is established, there are no longer any fork contexts to hold a reference to an OutgoingTransaction. So, when a re-INVITE goes through the media relay (e.g. to add video), an OutgoingTransaction is created on the fly to remember the ICE context (RelayedCall), except this transaction was freed before the request reached the forward module, which created a new OutgoingTransaction.
When the answer came through, that last OutgoingTransaction was notified, but did not have a pointer to the RelayedCall, and thus did not update the ICE state.
Later on, when an ICE re-INVITE is sent, the MediaRelay was in an invalid state and wrongly assumed it should add its 'relay' candidates.
Closes FLEXISIP-355