fix(channelserver): validate client binary blobs before saving

- Reject BinaryType outside 1-5 in SetUserBinary to prevent
  dynamic column name with unchecked client input
- Check rengoku payload length before DB write and fixed-offset
  reads to prevent panic on short payloads
- Require MercData >= 4 bytes before ReadUint32 to prevent panic

Ref: Mezeporta/Erupe#158

server/channelserver/handlers_rengoku.go

@@ -16,6 +16,11 @@ func handleMsgMhfSaveRengokuData(s *Session, p mhfpacket.MHFPacket) {
 	// saved every floor on road, holds values such as floors progressed, points etc.
 	// can be safely handled by the client
 	pkt := p.(*mhfpacket.MsgMhfSaveRengokuData)
+	if len(pkt.RawDataPayload) < 91 {
+		s.logger.Warn("Rengoku payload too short", zap.Int("len", len(pkt.RawDataPayload)))
+		doAckSimpleSucceed(s, pkt.AckHandle, make([]byte, 4))
+		return
+	}
 	dumpSaveData(s, pkt.RawDataPayload, "rengoku")
 	_, err := s.server.db.Exec("UPDATE characters SET rengokudata=$1 WHERE id=$2", pkt.RawDataPayload, s.charID)
 	if err != nil {