Fixed a bug preventing dual side packets from working in 1.6.4.

An example of a dual side packet is Packet101CloseWindow, which is 
sent by the server when it forces a inventory window to close, or 
by the client to the server when the player voluntarily closes it.

The bug prevented the client-side of a dual side packet from being 
recognized in 1.6.4. Thanks to Shevchikden for discovering the bug, 
and finding the correct build number where it was introduced.