avformat/matroskadec: Move AVBufferRef instead of copying, fix memleak

EBML binary elements are already made reference-counted when read;
so when populating the AVStream.attached_pic, one does not need to
allocate a new buffer for the data; instead the current code just
creates a new reference to the underlying AVBuffer. But this can be
improved even further: Just move the already existing reference.

This also fixes a memleak that happens upon error because
matroska_read_close has not been called in this scenario.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
(cherry picked from commit cbe336c9e81e2d9de3a18abef887c9255a9b9da5)

libavformat/matroskadec.c

@@ -2938,9 +2938,8 @@ static int matroska_read_header(AVFormatContext *s)
                 st->codecpar->codec_type = AVMEDIA_TYPE_VIDEO;
 
                 av_init_packet(pkt);
-                pkt->buf = av_buffer_ref(attachments[j].bin.buf);
-                if (!pkt->buf)
-                    return AVERROR(ENOMEM);
+                pkt->buf          = attachments[j].bin.buf;
+                attachments[j].bin.buf = NULL;
                 pkt->data         = attachments[j].bin.data;
                 pkt->size         = attachments[j].bin.size;
                 pkt->stream_index = st->index;