[gdk-pixbuf] Avoid integer overflow in gdk_pixbuf_add_alpha

[Matthias Clasen <matthiasc src gnome org>]

commit ca3c56421c075e729750cf80c3438b283232cce8
Author: Matthias Clasen <mclasen redhat com>
Date:   Mon Aug 24 15:20:08 2015 -0400

    Avoid integer overflow in gdk_pixbuf_add_alpha
    
    Same as before: don't do ptr = base + y * rowstride if y and
    rowstride are integers.
    
    This should fix http://bugzilla.gnome/org/753569

 gdk-pixbuf/gdk-pixbuf-util.c |   18 +++++++++---------
 1 files changed, 9 insertions(+), 9 deletions(-)
---
diff --git a/gdk-pixbuf/gdk-pixbuf-util.c b/gdk-pixbuf/gdk-pixbuf-util.c
index 6abe9b9..3600450 100644
--- a/gdk-pixbuf/gdk-pixbuf-util.c
+++ b/gdk-pixbuf/gdk-pixbuf-util.c
@@ -67,6 +67,8 @@ gdk_pixbuf_add_alpha (const GdkPixbuf *pixbuf,
        int x, y;
        const guint8 *src_pixels;
        guint8 *ret_pixels;
+       const guchar *src;
+       guchar *dest;
 
        g_return_val_if_fail (GDK_IS_PIXBUF (pixbuf), NULL);
        g_return_val_if_fail (pixbuf->colorspace == GDK_COLORSPACE_RGB, NULL);
@@ -85,20 +87,18 @@ gdk_pixbuf_add_alpha (const GdkPixbuf *pixbuf,
        } else {
                 new_pixbuf = gdk_pixbuf_new (GDK_COLORSPACE_RGB, TRUE, 8, pixbuf->width, pixbuf->height);
         }
-        
+
        if (!new_pixbuf)
                return NULL;
 
        ret_pixels = gdk_pixbuf_get_pixels (new_pixbuf);
 
-       for (y = 0; y < pixbuf->height; y++) {
-               const guchar *src;
-               guchar *dest;
+       for (y = 0; y < pixbuf->height; y++, src_pixels += pixbuf->rowstride, ret_pixels += 
new_pixbuf->rowstride) {
                guchar tr, tg, tb;
 
-               src = src_pixels + y * pixbuf->rowstride;
-               dest = ret_pixels + y * new_pixbuf->rowstride;
-                
+                src = src_pixels;
+                dest = ret_pixels;
+
                 if (pixbuf->has_alpha) {
                         /* Just subst color, we already copied everything else */
                         for (x = 0; x < pixbuf->width; x++) {
@@ -107,12 +107,12 @@ gdk_pixbuf_add_alpha (const GdkPixbuf *pixbuf,
                                 src += 4;
                                 dest += 4;
                         }
-                } else {                        
+                } else {
                         for (x = 0; x < pixbuf->width; x++) {
                                 tr = *dest++ = *src++;
                                 tg = *dest++ = *src++;
                                 tb = *dest++ = *src++;
-                                
+
                                 if (substitute_color && tr == r && tg == g && tb == b)
                                         *dest++ = 0;
                                 else