Crash when holding the call after receiving SDP answer with multiple codecs (thanks Cyril GY for the report)

[bennylp]

pjsua as caller, with the following scenario:

1. --> INVITE
2. <-- 18x (with SDP and multiple codecs/format list in m= line)
3. --> UPDATE (pjsip sends UPDATE to lock to one codec)
4. <-- 200/OK to UPDATE
5. <-- 200/OK to INVITE
6. --> ACK
7. --> UPDATE --> this will crash

The UPDATE is sent to instruct callee to use only one particular codec. This feature was implemented in #476.

PJSIP crashes on the last step above when it tries to build SDP to be sent to remote host. This is because the SDP in it's cache is corrupted, presumably because it was built with the wrong flip-flop pool.