magnifier: Sanity check coordinates in scrollContentsTo

We may receive a11y events with bogus coordinates (eg. x/y = minint),
so ensure the coordinates are sane before scrolling there.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/984

js/ui/magnifier.js

@@ -1168,6 +1168,10 @@ var ZoomRegion = class ZoomRegion {
      * @param {number} y: The y-coord of the point to center on.
      */
     scrollContentsTo(x, y) {
+        if (x < 0 || x > global.screen_width ||
+            y < 0 || y > global.screen_height)
+            return;
+
         this._clearScrollContentsTimer();
 
         this._followingCursor = false;