[gtk/ebassi/for-master] Blend nodes might have NULL top/bottom nodes

[Emmanuele Bassi <ebassi src gnome org>]

commit 247fc2e47190ed8cb69cc51a3589b1dc0037142c
Author: Emmanuele Bassi <ebassi gnome org>
Date:   Thu Nov 26 00:21:11 2020 +0000

    Blend nodes might have NULL top/bottom nodes
    
    We have code in place to handle a NULL node in the state when dealing
    with blend nodes, but we don't always check for NULL, which leads to
    warnings in the CSS Blend modes demo.

 gtk/gtksnapshot.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)
---
diff --git a/gtk/gtksnapshot.c b/gtk/gtksnapshot.c
index e3b3538c2f..40122f6338 100644
--- a/gtk/gtksnapshot.c
+++ b/gtk/gtksnapshot.c
@@ -1178,7 +1178,9 @@ gtk_snapshot_collect_blend_top (GtkSnapshot      *snapshot,
   GdkRGBA transparent = { 0, 0, 0, 0 };
 
   top_node = gtk_snapshot_collect_default (snapshot, state, nodes, n_nodes);
-  bottom_node = gsk_render_node_ref (state->data.blend.bottom_node);
+  bottom_node = state->data.blend.bottom_node != NULL
+              ? gsk_render_node_ref (state->data.blend.bottom_node)
+              : NULL;
 
   g_assert (top_node != NULL || bottom_node != NULL);
 
@@ -1199,7 +1201,7 @@ gtk_snapshot_collect_blend_top (GtkSnapshot      *snapshot,
 static void
 gtk_snapshot_clear_blend_top (GtkSnapshotState *state)
 {
-  gsk_render_node_unref (state->data.blend.bottom_node);
+  g_clear_pointer (&(state->data.blend.bottom_node), gsk_render_node_unref);
 }
 
 static GskRenderNode *