[vala] Fix source filenames having non-utf8 path components

[Luca Bruno <lucabru src gnome org>]

commit 2ac1eacaba7753913a613d6eca2186bfe68b3fee
Author: Luca Bruno <lucabru src gnome org>
Date:   Wed Mar 4 16:16:00 2015 +0100

    Fix source filenames having non-utf8 path components
    
    Fixes bug 742071

 vala/valacodecontext.vala |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/vala/valacodecontext.vala b/vala/valacodecontext.vala
index b9ac824..612ca2a 100644
--- a/vala/valacodecontext.vala
+++ b/vala/valacodecontext.vala
@@ -641,7 +641,8 @@ public class Vala.CodeContext {
                                        rpath += Path.DIR_SEPARATOR_S;
                                }
 
-                               rpath += start.substring (0, len);
+                               // don't use len, substring works on bytes
+                               rpath += start.substring (0, (long)((char*)end - (char*)start));
                        }
                }