Make avfilter_start_frame() invoke avfilter_get_video_buffer() on the
link rather than avfilter_default_get_video_buffer().

This is required as the buffer requested may be greater than the
buffer allocated locally by avfilter_default_get_video_buffer(), for
example if in filterchain there is a pad filter (like in "fifo,pad").

In that case the pad filter will try to write beyond the data of the
allocated buffer, resulting in a crash.

Originally committed as revision 24895 to svn://svn.ffmpeg.org/ffmpeg/trunk
