mpeg12dec: unref discarded picture from extradata

Otherwise another frame gets referenced into picture, triggering an assert
(from commit 13aae8) in av_frame_ref.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

libavcodec/mpeg12dec.c

@@ -2808,6 +2808,7 @@ static int mpeg_decode_frame(AVCodecContext *avctx, void *data,
                             avctx->extradata, avctx->extradata_size);
         if (*got_output) {
             av_log(avctx, AV_LOG_ERROR, "picture in extradata\n");
+            av_frame_unref(picture);
             *got_output = 0;
         }
         s->extradata_decoded = 1;