[gnome-flashback] common: fix unintentional integer overflow warnings

[Alberts Muktupāvels <muktupavels src gnome org>]

commit e7d0e92e2f46491a08a2b0c8b1f54a9f6366c3ab
Author: Alberts Muktupāvels <alberts muktupavels gmail com>
Date:   Sat Mar 5 13:40:45 2022 +0200

    common: fix unintentional integer overflow warnings
    
    Coverity CID: #1502894, #1514741, #1514742, #1514743

 gnome-flashback/libcommon/gf-bg.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)
---
diff --git a/gnome-flashback/libcommon/gf-bg.c b/gnome-flashback/libcommon/gf-bg.c
index d9f5ffe..67cb3a9 100644
--- a/gnome-flashback/libcommon/gf-bg.c
+++ b/gnome-flashback/libcommon/gf-bg.c
@@ -1241,12 +1241,12 @@ pixbuf_average_value (GdkPixbuf *pixbuf,
                                a = *p++;
                                
                                a_total += a;
-                               r_total += r * a;
-                               g_total += g * a;
-                               b_total += b * a;
+                               r_total += (guint64) r * a;
+                               g_total += (guint64) g * a;
+                               b_total += (guint64) b * a;
                        }
                }
-               dividend = height * width * 0xFF;
+               dividend = (guint64) height * width * 0xFF;
                a_total *= 0xFF;
        } else {
                for (row = 0; row < height; row++) {
@@ -1261,7 +1261,7 @@ pixbuf_average_value (GdkPixbuf *pixbuf,
                                b_total += b;
                        }
                }
-               dividend = height * width;
+               dividend = (guint64) height * width;
                a_total = dividend * 0xFF;
        }