fix fragment length when checksums are used

peer.c

@@ -108,6 +108,8 @@ enet_peer_send (ENetPeer * peer, enet_uint8 channelID, ENetPacket * packet)
      return -1;
 
    fragmentLength = peer -> mtu - sizeof (ENetProtocolHeader) - sizeof (ENetProtocolSendFragment);
+   if (peer -> host -> checksum != NULL)
+     fragmentLength -= sizeof(enet_uint32);
 
    if (packet -> dataLength > fragmentLength)
    {