Fixed #1079 (Media transport should be kept alive during double-hold scenario). Details:
 - now the stream will be destroyed but the media transport will be kept alive during doublehold scenario
 - small fix in SRTP to also negotiate crypto even when the media is marked as inactive, otherwise it's possible that an "optional" endpoint would create RTP/AVP offer and send it to "mandatory" endpoint, which would be rejected and cause the media port to be set to zero


git-svn-id: https://svn.pjsip.org/repos/pjproject/trunk@3219 74dad513-b988-da41-8d7b-12977e46ad98
3 files changed