[gtk/popup-shadow-width] css: Avoid an invalid read

[Matthias Clasen <matthiasc src gnome org>]

commit f128ba24a32bf99ed5b887dcce3edb17e1e2bb2d
Author: Matthias Clasen <mclasen redhat com>
Date:   Wed Feb 10 19:13:36 2021 -0500

    css: Avoid an invalid read
    
    This was broken in ea7185bdb1ba423495340a9b880e619a861064fd.
    
    Pointed out by Christian Hergert.

 gtk/gtkcssfiltervalue.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)
---
diff --git a/gtk/gtkcssfiltervalue.c b/gtk/gtkcssfiltervalue.c
index c1c281c1e2..ed334979cb 100644
--- a/gtk/gtkcssfiltervalue.c
+++ b/gtk/gtkcssfiltervalue.c
@@ -991,10 +991,13 @@ gtk_css_filter_value_pop_snapshot (const GtkCssValue *filter,
       if (i < j)
         gtk_snapshot_pop (snapshot);
 
-      if (filter->filters[j].type == GTK_CSS_FILTER_BLUR)
-        gtk_snapshot_pop (snapshot);
-      else if (filter->filters[j].type == GTK_CSS_FILTER_DROP_SHADOW)
-        gtk_css_shadow_value_pop_snapshot (filter->filters[j].drop_shadow.value, snapshot);
+      if (j < filter->n_filters)
+        {
+          if (filter->filters[j].type == GTK_CSS_FILTER_BLUR)
+            gtk_snapshot_pop (snapshot);
+          else if (filter->filters[j].type == GTK_CSS_FILTER_DROP_SHADOW)
+            gtk_css_shadow_value_pop_snapshot (filter->filters[j].drop_shadow.value, snapshot);
+        }
 
       i = j + 1;
     }