free private streamcontext when closing demuxer, fix memory leak, patch by Art Clarke, aclarke at vlideshow dot com
free private streamcontext when closing demuxer, fix memory leak, patch by Art Clarke, aclarke at vlideshow dot com

git-svn-id: a4d7c1866f8397a4106e0b57fc4fbf792bbdaaaf@13800 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

prajnashi/ffmpeg,prajnashi/ffmpeg,prajnashi/ffmpeg,prajnashi/ffmpeg