Use Helpers.SafeIdentifier when sending parameters to marshaling to deal with invalid identifiers.

src/Generator/Generators/CSharp/CSharpTextTemplate.cs

@@ -922,7 +922,7 @@ namespace CppSharp.Generators.CSharp
             {
                 var ctx = new CSharpMarshalContext(Driver)
                 {
-                    ReturnVarName = param.Name,
+                    ReturnVarName = Helpers.SafeIdentifier(param.Name),
                     ReturnType = param.QualifiedType
                 };