Fixed crash when file path of a unit was invalid.

src/Generator/Generator.cs

@@ -71,7 +71,7 @@ namespace CppSharp.Generators
                 if (!unit.IsGenerated || !unit.HasDeclarations)
                     continue;
 
-                if (unit.IsSystemHeader)
+                if (unit.IsSystemHeader || !unit.IsValid)
                     continue;
 
                 var templates = Generate(unit);

src/Generator/Passes/CleanUnitPass.cs

@@ -18,10 +18,13 @@ namespace CppSharp.Passes
                 
             // Try to get an include path that works from the original include
             // directories paths.
-
+            if (unit.IsValid)
+            {
                 unit.IncludePath = GetIncludePath(unit.FilePath);
                 return true;
             }
+            return false;
+        }
 
         string GetIncludePath(string filePath)
         {