Prevent GAME table rows for non-game channels

[dbisdorf]

The get_prefix() function, which returns the user-configured command prefix, will create a GAME row for every channel that submits a message, regardless of whether that message was intended for the bot.

The function should not create a GAME row if no row exists for the server/channel that triggered the function. When no row exists, the function should return the "$" default command prefix.