res_pjsip_session: Preserve label on incoming re-INVITE.

When a re-INVITE is received we create a new set of
streams that are then swapped in as the active streams.
We did not preserve the SDP label from the previous
streams, resulting in the label getting lost.

This change ensures that if an SDP label is present
on the previous stream then it is set on the new stream.

ASTERISK-28953

Change-Id: I9dd63b88b562fe96ce5c791a3dae5bcaca258445