[gdk-pixbuf] thumbnailer: Update from gnome-thumbnailer-skeleton master

[Philip Withnall <pwithnall src gnome org>]

commit ab921f069d2624de547c9a15252fb278de2baf13
Author: Philip Withnall <philip tecnocode co uk>
Date:   Sat Jan 7 17:03:34 2017 +0000

    thumbnailer: Update from gnome-thumbnailer-skeleton master
    
    https://github.com/hadess/gnome-thumbnailer-skeleton/commits/master
    
    Add some assertions to help Coverity
    
    From looking at the code, these assertions should always hold already,
    but it clarifies the code and helps static analysis if we add them
    explicitly.
    
    (n_pixels == 0) is only true if (s_x1 == s_x2 || s_y1 == s_y2), which is
    only true if (dx == 0 || dy == 0). This cannot be true, because dx and
    dy are the result of dividing source_[width|height] by
    dest_[width|height], and there’s already an assertion that the former
    are greater or equal to the latter.
    
    Coverity CID: 1388528
    
    https://bugzilla.gnome.org/show_bug.cgi?id=776990

 thumbnailer/gnome-thumbnailer-skeleton.c |    7 ++++++-
 1 files changed, 6 insertions(+), 1 deletions(-)
---
diff --git a/thumbnailer/gnome-thumbnailer-skeleton.c b/thumbnailer/gnome-thumbnailer-skeleton.c
index 10b8d73..d686432 100644
--- a/thumbnailer/gnome-thumbnailer-skeleton.c
+++ b/thumbnailer/gnome-thumbnailer-skeleton.c
@@ -88,6 +88,9 @@ gnome_desktop_thumbnail_scale_down_pixbuf (GdkPixbuf *pixbuf,
        dy = ddy.quot;
        dy_frac = ddy.rem;
 
+       g_assert (dx >= 1);
+       g_assert (dy >= 1);
+
        has_alpha = gdk_pixbuf_get_has_alpha (pixbuf);
        source_rowstride = gdk_pixbuf_get_rowstride (pixbuf);
        src_pixels = gdk_pixbuf_get_pixels (pixbuf);
@@ -146,7 +149,9 @@ gnome_desktop_thumbnail_scale_down_pixbuf (GdkPixbuf *pixbuf,
                                }
                                src += source_rowstride;
                        }
-                       
+
+                       g_assert (n_pixels > 0);
+
                        if (has_alpha) {
                                if (a != 0) {
                                        *dest++ = r / a;