background: don't choke on huge slide show durations

if a slideshow file has a really large duration we'll currently
throw an exception.

This bug is aggravated by the fact that some versions of
gnome-desktop use UINT_MAX as a sentinel value to mean,
"don't ever update slide".

This commit treats durations that would overflow as infinitely
long.

https://bugzilla.gnome.org/show_bug.cgi?id=696157

js/ui/background.js

@@ -503,6 +503,10 @@ const Background = new Lang.Class({
 
         let interval = Math.max(ANIMATION_MIN_WAKEUP_INTERVAL * 1000,
                                 timePerStep);
+
+        if (interval > GLib.MAXUINT32)
+            return;
+
         this._updateAnimationTimeoutId = GLib.timeout_add(GLib.PRIORITY_DEFAULT,
                                                       interval,
                                                       Lang.bind(this, function() {