decoder/ogg_codec: return UNKNOWN on error

src/decoder/ogg_codec.c

@@ -33,7 +33,7 @@ ogg_codec_detect(struct decoder *decoder, struct input_stream *is)
 	unsigned char buf[41];
 	size_t r = decoder_read(decoder, is, buf, sizeof(buf));
 	if (r < sizeof(buf) || memcmp(buf, "OggS", 4) != 0)
-		return OGG_CODEC_VORBIS;
+		return OGG_CODEC_UNKNOWN;
 
 	if ((memcmp(buf + 29, "FLAC", 4) == 0 &&
 	     memcmp(buf + 37, "fLaC", 4) == 0) ||

src/decoder/ogg_codec.h

@@ -27,6 +27,7 @@
 #include "decoder_api.h"
 
 enum ogg_codec {
+	OGG_CODEC_UNKNOWN,
 	OGG_CODEC_VORBIS,
 	OGG_CODEC_FLAC,
 };