fix segfault if flush is called before the first frame (I think that's it, and this fixes it)
fix segfault if flush is called before the first frame
(I think that's it, and this fixes it)


git-svn-id: a4d7c1866f8397a4106e0b57fc4fbf792bbdaaaf@4335 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

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