decoder/opus: fix mistyped LoadEOSPacket() return value

src/decoder/plugins/OpusDecoderPlugin.cxx

@@ -188,7 +188,7 @@ LoadEOSPacket(InputStream &is, Decoder *decoder, int serialno,
 		/* we do this for local files only, because seeking
 		   around remote files is expensive and not worth the
 		   troubl */
-		return -1;
+		return false;
 
 	const auto old_offset = is.GetOffset();