libavfilter: Support the forks ABI for buffer sinks

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>
