Fixed #1797: Clean up provisional media is executed when there is an ongoing SDP negotiation.
Scenario:
1. re-invite got 491, state tsx changed to completed
2. cleanup prov med executed
3. re-invite again with video
4. timer tsx to shift tsx no 1, from completed -> terminated trigger provisional cleanup 
5. provisional video transport in no 3 got cleanup



git-svn-id: http://svn.pjsip.org/repos/pjproject@4956 74dad513-b988-da41-8d7b-12977e46ad98