[Resolver] Fixed unexpected resolver crash.

Test Case:
Action foo = f => { f.FirstOrDefault(l=>); };

ICSharpCode.NRefactory.CSharp/Resolver/ResolveVisitor.cs

@@ -1,4 +1,4 @@
-// Copyright (c) AlphaSierraPapa for the SharpDevelop Team
+// Copyright (c) AlphaSierraPapa for the SharpDevelop Team
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software
@@ -2525,6 +2525,11 @@ namespace ICSharpCode.NRefactory.CSharp.Resolver
 			Log.Indent();
 			while (undecidedLambdas.Count > 0) {
 				LambdaBase lambda = undecidedLambdas[0];
+				// may happen caused by parse error l =>
+				if (lambda.LambdaExpression == null) {
+					undecidedLambdas.Remove (lambda);
+					continue;
+				}
 				ResolveParentForConversion(lambda.LambdaExpression);
 				if (lambda.IsUndecided) {
 					// Lambda wasn't merged by resolving its parent -> enforce merging