res_pjsip_session: properly handle SDP from a forked call with early media

In handle_negotiated_sdp(), use session->active_media_state when
session->pending_media_state is empty.  The 200's SDP should be fed into
handle_negotiated_sdp_session_media() together with the already negotiated
state, which is now in session->active_media_state instead.  Only if both
the session's pending and active media are empty should
handle_negotiated_sdp() abort.

ASTERISK-27441

Change-Id: If0d5150ffe6f38d8a854831fef37942258d4629c