Merge pull request #428 from stsundermann/master

Don't visit invalid units

src/Generator/Passes/Pass.cs

@@ -30,6 +30,9 @@ namespace CppSharp.Passes
 
         public virtual bool VisitTranslationUnit(TranslationUnit unit)
         {
+            if (!unit.IsValid)
+                return false;
+
             if (unit.IsSystemHeader)
                 return false;