[perl-Gtk2] Document that Gtk2::Gdk::Drawable->get_colormap() can return undef

[Torsten Schönfeld <tsch src gnome org>]

commit a80b29069131e921566014ec68ee2029823d8a6d
Author: Kevin Ryde <user42 zip com au>
Date:   Thu Jul 8 11:00:41 2010 +1000

    Document that Gtk2::Gdk::Drawable->get_colormap() can return undef
    
    gdk_drawable_get_colormap() can return NULL, on a pixmap which has never had a
    colormap set.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=623800

 xs/GdkDrawable.xs |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/xs/GdkDrawable.xs b/xs/GdkDrawable.xs
index 445bdcb..9ad6181 100644
--- a/xs/GdkDrawable.xs
+++ b/xs/GdkDrawable.xs
@@ -48,7 +48,7 @@ gdk_drawable_set_colormap (drawable, colormap)
 	GdkColormap *colormap
 
  ## GdkColormap* gdk_drawable_get_colormap (GdkDrawable *drawable)
-GdkColormap*
+GdkColormap_ornull *
 gdk_drawable_get_colormap (drawable)
 	GdkDrawable *drawable