lavf/mkv: Only skip prores header if the packet is large enough.

Fixes a possible endless loop.

libavformat/matroskaenc.c

@@ -1575,7 +1575,7 @@ static void mkv_write_block(AVFormatContext *s, AVIOContext *pb,
     } else
         data = pkt->data;
 
-    if (codec->codec_id == AV_CODEC_ID_PRORES) {
+    if (codec->codec_id == AV_CODEC_ID_PRORES && size >= 8) {
         /* Matroska specification requires to remove the first QuickTime atom
          */
         size  -= 8;