[dia] Sprinkle g_return_val_if_fail() to avoid crash on wrong API usage

[Hans Breuer <hans src gnome org>]

commit c28fab5161848d4830418d77d4e538d54998df66
Author: Hans Breuer <hans breuer org>
Date:   Fri Sep 7 20:24:07 2012 +0200

    Sprinkle g_return_val_if_fail() to avoid crash on wrong API usage

 lib/dia_image.c |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)
---
diff --git a/lib/dia_image.c b/lib/dia_image.c
index 8f99ac7..ae20386 100644
--- a/lib/dia_image.c
+++ b/lib/dia_image.c
@@ -313,6 +313,7 @@ dia_image_save(DiaImage *image, const gchar *filename)
 int 
 dia_image_width(const DiaImage *image)
 {
+  g_return_val_if_fail (image != NULL, 0);
   return gdk_pixbuf_get_width(image->image);
 }
 
@@ -323,6 +324,7 @@ dia_image_width(const DiaImage *image)
 int 
 dia_image_height(const DiaImage *image)
 {
+  g_return_val_if_fail (image != NULL, 0);
   return gdk_pixbuf_get_height(image->image);
 }
 
@@ -333,6 +335,7 @@ dia_image_height(const DiaImage *image)
 int
 dia_image_rowstride(const DiaImage *image)
 {
+  g_return_val_if_fail (image != NULL, 0);
   return gdk_pixbuf_get_rowstride(image->image);
 }
 /** Direct const access to the underlying GdkPixbuf
@@ -423,6 +426,7 @@ dia_image_mask_data(const DiaImage *image)
 const guint8 *
 dia_image_rgba_data(const DiaImage *image)
 {
+  g_return_val_if_fail (image != NULL, 0);
   if (gdk_pixbuf_get_has_alpha(image->image)) {
     const guint8 *pixels = gdk_pixbuf_get_pixels(image->image);