Only insert the icpf atom in ProRes mkv packets if it isn't already present.

The patch allows to read broken mkv files containing ProRes as written by
the Haali muxer.
Fixes a part of ticket #3434.

libavformat/matroskadec.c

@@ -2545,7 +2545,8 @@ static int matroska_parse_frame(MatroskaDemuxContext *matroska,
         pkt_data = wv_data;
     }
 
-    if (st->codec->codec_id == AV_CODEC_ID_PRORES)
+    if (st->codec->codec_id == AV_CODEC_ID_PRORES &&
+        AV_RB32(&data[4]) != MKBETAG('i', 'c', 'p', 'f'))
         offset = 8;
 
     pkt = av_mallocz(sizeof(AVPacket));
@@ -2556,7 +2557,7 @@ static int matroska_parse_frame(MatroskaDemuxContext *matroska,
         goto fail;
     }
 
-    if (st->codec->codec_id == AV_CODEC_ID_PRORES) {
+    if (st->codec->codec_id == AV_CODEC_ID_PRORES && offset == 8) {
         uint8_t *buf = pkt->data;
         bytestream_put_be32(&buf, pkt_size);
         bytestream_put_be32(&buf, MKBETAG('i', 'c', 'p', 'f'));