fix some dumb (nonfatal) crashes in the editor with nothing loaded

OpenRA.Editor/Surface.cs

@@ -87,6 +87,8 @@ namespace OpenRA.Editor
 		{
 			base.OnMouseWheel(e);
 
+			if (Map == null) return;
+
 			Zoom *= e.Delta > 0 ? 4.0f / 3.0f : .75f;
 
 			Invalidate();
@@ -114,6 +116,8 @@ namespace OpenRA.Editor
 		{
 			base.OnMouseMove(e);
 
+			if (Map == null) return;
+
 			var oldMousePos = MousePos;
 			MousePos = new int2(e.Location);
 
@@ -332,6 +336,8 @@ namespace OpenRA.Editor
 		{
 			base.OnMouseDown(e);
 
+			if (Map == null) return;
+
 			if (!IsPanning)
 			{
 				if (e.Button == MouseButtons.Right) Erase();