Implement av_buffersink_read() and av_buffersink_read_samples()
for ffmpeg's version of buffersink.
With this change, avconv linked against ffmpeg's libraries passes
the same number of tests whether it uses ffbuffersink or
buffersink_old.
With this change avconv compiled against libav and linked to ffmpegs libs
will run through the whole fate testsuite without any crashes.
857 tests pass, the remaining tests fail one way or another, which is
to be expected as avconv is not a drop in replacement for ffmpeg
The testsuite used was the ffmpeg fate testsuite, not libavs.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>