Fix OpenRA.Lint throwing because "perf" log channel does not exist

Slightly modified the Log class to allow dummy channels that don't
write anywhere, then use it on Lint because map loading writes to
the "perf" channel.

OpenRA.Game/Support/Log.cs

@@ -46,6 +46,12 @@ namespace OpenRA
 		{
 			if (Channels.ContainsKey(channelName)) return;
 
+			if (string.IsNullOrEmpty(baseFilename))
+			{
+				Channels.Add(channelName, new ChannelInfo());
+				return;
+			}
+
 			foreach (var filename in FilenamesForChannel(channelName, baseFilename))
 				try
 				{
@@ -70,6 +76,9 @@ namespace OpenRA
 			if (!Channels.TryGetValue(channel, out info))
 				throw new Exception("Tried logging to non-existant channel " + channel);
 
+			if (info.Writer == null)
+				return;
+
 			info.Writer.WriteLine(format, args);
 		}
 	}

OpenRA.Lint/YamlChecker.cs

@@ -6,6 +6,9 @@
  * as published by the Free Software Foundation. For more information,
  * see COPYING.
  */
+using System.IO;
+
+
 #endregion
 
 using System;
@@ -41,6 +44,8 @@ namespace OpenRA.Lint
 
 			try
 			{
+				Log.AddChannel("perf", null);
+
 				var options = args.Where(a => a.StartsWith("-"));
 				var mod = args.Where(a => !options.Contains(a)).First();
 				var map = args.Where(a => !options.Contains(a)).Skip(1).FirstOrDefault();