[gtk+] treeviewcolumn: Don't leak button

[Timm Bäder <baedert src gnome org>]

commit 2c28c627c1527d4ee2dfc4d3805f7afc482049e2
Author: Timm Bäder <mail baedert org>
Date:   Fri Oct 6 17:23:34 2017 +0200

    treeviewcolumn: Don't leak button
    
    We g_object_ref_sink the button in _create_button, so we need to unref
    it ourselves.

 gtk/gtktreeviewcolumn.c |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/gtk/gtktreeviewcolumn.c b/gtk/gtktreeviewcolumn.c
index 3f2bb97..60ddef0 100644
--- a/gtk/gtktreeviewcolumn.c
+++ b/gtk/gtktreeviewcolumn.c
@@ -1336,6 +1336,7 @@ _gtk_tree_view_column_unset_tree_view (GtkTreeViewColumn *column)
   if (priv->tree_view)
     {
       gtk_container_remove (GTK_CONTAINER (priv->tree_view), priv->button);
+      g_clear_object (&priv->button);
     }
   if (priv->property_changed_signal)
     {