Fix bug with just typing "/w ".

Yeah, whitespace seems to matter.
Log before this fix:
[10/25/13 23:58:17] [Server API] Warning: Plugin "AntiSpam" has had an unhandled exception thrown by one of its ServerChat handlers: 
System.ArgumentOutOfRangeException: startIndex cannot be larger than length of string.
Parameter name: startIndex
   at System.String.InternalSubStringWithChecks(Int32 startIndex, Int32 length, Boolean fAlwaysCopy)
   at AntiSpam.AntiSpam.OnChat(ServerChatEventArgs e)
   at TerrariaApi.Server.HandlerCollection`1.Invoke(ArgsType args)