shn: do not allow seeking in the raw shn demuxer.

The demuxer does not read the seektable, a parser is not possible without a
full decode, and no shorten decoder can handle random seeking because it needs
side info from the seektable.

libavformat/rawdec.c

@@ -244,7 +244,7 @@ AVInputFormat ff_shorten_demuxer = {
     .long_name      = NULL_IF_CONFIG_SMALL("raw Shorten"),
     .read_header    = ff_raw_audio_read_header,
     .read_packet    = ff_raw_read_partial_packet,
-    .flags= AVFMT_GENERIC_INDEX,
+    .flags          = AVFMT_NOBINSEARCH | AVFMT_NOGENSEARCH | AVFMT_NO_BYTE_SEEK,
     .extensions = "shn",
     .value = CODEC_ID_SHORTEN,
 };