ffmpeg: Fix bitstream filters manipulating AVCodecContext

Fixes Ticket3715
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg.c

@@ -575,7 +575,7 @@ static void close_all_output_streams(OutputStream *ost, OSTFinished this_stream,
 static void write_frame(AVFormatContext *s, AVPacket *pkt, OutputStream *ost)
 {
     AVBitStreamFilterContext *bsfc = ost->bitstream_filters;
-    AVCodecContext          *avctx = ost->enc_ctx;
+    AVCodecContext          *avctx = ost->st->codec;
     int ret;
 
     if ((avctx->codec_type == AVMEDIA_TYPE_VIDEO && video_sync_method == VSYNC_DROP) ||