[vala] codegen: fix criticals when duplicating unowned instnaces

[Luca Bruno <lucabru src gnome org>]

commit bc4fc3dcd6bd5e9b266c3158a033acfa8e64ee89
Author: Luca Bruno <lucabru src gnome org>
Date:   Sat Jan 30 14:11:04 2016 +0100

    codegen: fix criticals when duplicating unowned instnaces
    
    Fixes bug 632174

 codegen/valaccodebasemodule.vala |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/codegen/valaccodebasemodule.vala b/codegen/valaccodebasemodule.vala
index cda8912..6f4fc6c 100644
--- a/codegen/valaccodebasemodule.vala
+++ b/codegen/valaccodebasemodule.vala
@@ -2622,7 +2622,7 @@ public abstract class Vala.CCodeBaseModule : CodeGenerator {
                                dup_function = get_ccode_ref_function ((ObjectTypeSymbol) type.data_type);
                                if (type.data_type is Interface && dup_function == null) {
                                        Report.error (source_reference, "missing class prerequisite for 
interface `%s', add GLib.Object to interface declaration if unsure".printf (type.data_type.get_full_name ()));
-                                       return null;
+                                       return new CCodeInvalidExpression();
                                }
                        } else if (cl != null && cl.is_immutable) {
                                // allow duplicates of immutable instances as for example strings
@@ -2646,7 +2646,7 @@ public abstract class Vala.CCodeBaseModule : CodeGenerator {
                        } else {
                                // duplicating non-reference counted objects may cause side-effects (and 
performance issues)
                                Report.error (source_reference, "duplicating %s instance, use unowned 
variable or explicitly invoke copy method".printf (type.data_type.name));
-                               return null;
+                               return new CCodeInvalidExpression();
                        }
 
                        return new CCodeIdentifier (dup_function);