matroskadec: fix handling of A_MS/ACM track with no extradata

Originally committed as revision 20131 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/matroskadec.c

@@ -1248,13 +1248,13 @@ static int matroska_read_header(AVFormatContext *s, AVFormatParameters *ap)
             codec_id = ff_codec_get_id(ff_codec_bmp_tags, track->video.fourcc);
             extradata_offset = 40;
         } else if (!strcmp(track->codec_id, "A_MS/ACM")
-                   && track->codec_priv.size >= 18
+                   && track->codec_priv.size >= 14
                    && track->codec_priv.data != NULL) {
             init_put_byte(&b, track->codec_priv.data, track->codec_priv.size,
                           URL_RDONLY, NULL, NULL, NULL, NULL);
             ff_get_wav_header(&b, st->codec, track->codec_priv.size);
             codec_id = st->codec->codec_id;
-            extradata_offset = 18;
+            extradata_offset = FFMIN(track->codec_priv.size, 18);
         } else if (!strcmp(track->codec_id, "V_QUICKTIME")
                    && (track->codec_priv.size >= 86)
                    && (track->codec_priv.data != NULL)) {