[gtk] rbtree: Fix removing last element

[Benjamin Otte <otte src gnome org>]

commit e10433770482047e0c522f8a0fbb5a320c156b04
Author: Benjamin Otte <otte redhat com>
Date:   Mon Jan 14 18:42:18 2019 +0100

    rbtree: Fix removing last element
    
    The refactoring of automatically updating tree->root when setting a
    node's parent works very well - unless all nodes get removed and no
    node's parent got updated.

 gtk/gtkrbtree.c | 5 +++++
 1 file changed, 5 insertions(+)
---
diff --git a/gtk/gtkrbtree.c b/gtk/gtkrbtree.c
index 4e52189233..85055b1cb4 100644
--- a/gtk/gtkrbtree.c
+++ b/gtk/gtkrbtree.c
@@ -708,6 +708,11 @@ gtk_rb_tree_remove (GtkRbTree *tree,
        p->right = x;
       gtk_rb_node_mark_dirty (p, TRUE);
     }
+  else
+    {
+      if (x == NULL)
+        tree->root = NULL;
+    }
 
   /* We need to clean up the validity of the tree.
    */