[dia] Improve robustness of dia_context_get_filename() use

[Hans Breuer <hans src gnome org>]

commit 72c3148b956a6c352b7c5fbaf6bfac758760f688
Author: Hans Breuer <hans breuer org>
Date:   Sun Jan 19 12:23:56 2014 +0100

    Improve robustness of dia_context_get_filename() use
    
    Not setting the filename on the context should not produce
    crashes. So don't return NULL with dia_context_get_filename()
    but a valid string. Should be more reliable than adding
    NULL pointer to all the users.

 lib/diacontext.c         |   12 ++++++++++--
 objects/standard/image.c |    2 +-
 2 files changed, 11 insertions(+), 3 deletions(-)
---
diff --git a/lib/diacontext.c b/lib/diacontext.c
index 60f1168..04dfcf8 100644
--- a/lib/diacontext.c
+++ b/lib/diacontext.c
@@ -129,12 +129,21 @@ dia_context_set_filename (DiaContext *context,
   context->filename = g_strdup (filename);
 }
 
+/*!
+ * \brief Get the filename previously set with dia_context_set_filename()
+ *
+ * Returns the filename previously set with dia_context_set_filename(). For convenience
+ * of the API user a valid string gets returned even if no filename is set.
+ *
+ * @param  context explicit this pointer
+ * @return the filename or "?" instead of NULL
+ */
 const char *
 dia_context_get_filename (DiaContext *context)
 {
   g_return_val_if_fail (context != NULL, "");
 
-  return context->filename;
+  return context->filename ? context->filename : "?";
 }
 
 void 
@@ -144,7 +153,6 @@ dia_context_add_message (DiaContext *context,
   gchar *msg;
   va_list args;
 
-
   g_return_if_fail (context != NULL);
 
   g_return_if_fail (context != NULL);
diff --git a/objects/standard/image.c b/objects/standard/image.c
index 23eefc8..2957464 100644
--- a/objects/standard/image.c
+++ b/objects/standard/image.c
@@ -655,7 +655,7 @@ image_save(Image *image, ObjectNode obj_node, DiaContext *ctx)
     if (g_path_is_absolute(image->file)) { /* Absolute pathname */
       diafile_dir = get_directory(dia_context_get_filename (ctx));
 
-      if (strncmp(diafile_dir, image->file, strlen(diafile_dir))==0) {
+      if (diafile_dir && strncmp(diafile_dir, image->file, strlen(diafile_dir))==0) {
        /* The image pathname has the dia file pathname in the begining */
        /* Save the relative path: */
        data_add_filename(new_attribute(obj_node, "file"),