Fix rare crash

Author: lenemter

src/BackgroundBlurEffect.vala

@@ -340,6 +340,11 @@ public class Gala.BackgroundBlurEffect : Clutter.Effect {
         var width = (int) actor_box.get_width ();
         var height = (int) actor_box.get_height ();
 
+        if (width < 0 || height < 0) {
+            warning ("BackgroundBlurEffect: Couldn't update framebuffers, incorrect size");
+            return false;
+        }
+
         var downscale_factor = calculate_downscale_factor (width, height, real_blur_radius);
 
         var updated = update_actor_fbo (width, height, downscale_factor) && update_rounded_fbo (width, height, downscale_factor) && update_background_fbo (width, height);