ffmpeg: Use the buffer sinks poll instead of its input.

This fixes the case that the buffer sinks fifo contains frames which
would not show up on the poll from its input
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg.c

@@ -1918,7 +1918,7 @@ static int transcode_video(InputStream *ist, AVPacket *pkt, int *got_output, int
 
 #if CONFIG_AVFILTER
         if (ost->input_video_filter) {
-            frame_available = avfilter_poll_frame(ost->output_video_filter->inputs[0]);
+            frame_available = av_buffersink_poll_frame(ost->output_video_filter);
         }
         while (frame_available) {
             if (ost->output_video_filter) {
@@ -1951,7 +1951,7 @@ static int transcode_video(InputStream *ist, AVPacket *pkt, int *got_output, int
                 do_video_stats(output_files[ost->file_index].ctx, ost, frame_size);
 #if CONFIG_AVFILTER
             cont:
-            frame_available = ost->output_video_filter && avfilter_poll_frame(ost->output_video_filter->inputs[0]);
+            frame_available = ost->output_video_filter && av_buffersink_poll_frame(ost->output_video_filter);
             avfilter_unref_buffer(ost->picref);
         }
 #endif