Crash in SDP negotiator due to access to invalid memory space

[nanang]

INVITE session uses an alternating set of memory pools where the last used is reset after a cycle of SDP negotiation, this requires SDP negotiator to retain some of its internal states using the current pool in each SDP negotiation cycle, however there is a scenario where it fails to retain internal state initial_sdp:

1. normal SDP offer/answer + negotiation, this cycle uses pool 1
2. send reoffer using pjmedia_sdp_neg_send_local_offer(), this cycle uses pool 2, unfortunately initial_sdp is not retained by pjmedia_sdp_neg_send_local_offer(), so after SDP nego, pool 1 is reset and initial_sdp becomes invalid
3. receive reoffer, when answering using pjmedia_sdp_neg_set_local_answer(), the invalid initial_sdp is accessed.

Thanks Joshua Colp for the report and the patch.

[nanang]

In 5040:

Fix #1834: Fixed SDP negotiator to retain initial_sdp in pjmedia_sdp_neg_send_local_offer().