fix: set lobby to null on disconnect and prevent selecting lobby out of bounds

[jonandrewdavis]

Fixes an issue in the browser .gd script:

• When disconnecting and reconnecting you could select a lobby index higher than the available list
• When disconnecting and reconnecting, if you had a selection, it was not reset. This sets it to null.

Note: Just setting to null on disconnect should fix everything, but the extra check on if the lobby index is present before selecting could potentially prevent a crash I observed.

To test:

• 2 clients
• Connect
• Create Lobby
• Select lobby with 1 client
• Disconnect
• Reconnect

The selection will cause a crash on main. On this branch, no lobby will be in the selection once re-connected, which would be expected on a fresh session.