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);
 		}
 	}