[vala] codegen: Fix critical on symbols without source reference

[Jürg Billeter <juergbi src gnome org>]

commit bda74d748dcdb0ce94daafa03ee3dad16ffab1c2
Author: Jürg Billeter <j bitron ch>
Date:   Thu Oct 7 23:18:17 2010 +0200

    codegen: Fix critical on symbols without source reference

 codegen/valaccodebasemodule.vala |    4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)
---
diff --git a/codegen/valaccodebasemodule.vala b/codegen/valaccodebasemodule.vala
index fb52bf8..5c06956 100644
--- a/codegen/valaccodebasemodule.vala
+++ b/codegen/valaccodebasemodule.vala
@@ -486,7 +486,9 @@ public class Vala.CCodeBaseModule : CodeGenerator {
 		if (decl_space.add_declaration (name)) {
 			return true;
 		}
-		sym.source_reference.file.used = true;
+		if (sym.source_reference != null) {
+			sym.source_reference.file.used = true;
+		}
 		if (sym.external_package || (!decl_space.is_header && CodeContext.get ().use_header && !sym.is_internal_symbol ())) {
 			// add appropriate include file
 			foreach (string header_filename in sym.get_cheader_filenames ()) {