Check chain length Ogg FLAC file

When a metadata chain was read from an Ogg FLAC file containing no
metadata (but otherwise valid), an empty chain could be returned,
leading to null derefencing on trying to manipulate it. This commit
adds a check for the chain length

src/libFLAC/metadata_iterators.c

@@ -1360,6 +1360,12 @@ static FLAC__bool chain_read_ogg_cb_(FLAC__Metadata_Chain *chain, FLAC__IOHandle
 
 	chain->initial_length = chain_calculate_length_(chain);
 
+	if(chain->initial_length == 0) {
+		/* Ogg FLAC file must have at least streaminfo and vorbis comment */
+		chain->status = FLAC__METADATA_CHAIN_STATUS_BAD_METADATA;
+		return false;
+	}
+
 	return true;
 }