avcodec/avcodec: Simplify check for flushing of bsf

Just check for the existence of the bsf. This is equivalent to
the old criterion of the AVCodecContext being a decoder.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/avcodec.c

@@ -418,7 +418,7 @@ void avcodec_flush_buffers(AVCodecContext *avctx)
     avctx->pts_correction_last_pts =
     avctx->pts_correction_last_dts = INT64_MIN;
 
-    if (av_codec_is_decoder(avctx->codec))
+    if (avci->bsf)
         av_bsf_flush(avci->bsf);
 }