[glib] doc: clarify that _get_instance_private() is NULL-safe

[Xavier Claessens <xclaesse src gnome org>]

commit 32811598f384ac165a812b4dde8fb1ad4df2d420
Author: Xavier Claessens <xavier claessens collabora com>
Date:   Tue Sep 15 16:42:10 2015 -0400

    doc: clarify that _get_instance_private() is NULL-safe
    
    https://bugzilla.gnome.org/show_bug.cgi?id=755083

 gobject/gtype.h |    8 ++++++++
 1 files changed, 8 insertions(+), 0 deletions(-)
---
diff --git a/gobject/gtype.h b/gobject/gtype.h
index 611e637..f4cbf2f 100644
--- a/gobject/gtype.h
+++ b/gobject/gtype.h
@@ -1823,6 +1823,8 @@ guint     g_type_get_type_registration_serial (void);
  *   {
  *     MyObjectPrivate *priv = my_object_get_instance_private (obj);
  *
+ *     g_return_val_if_fail (MY_IS_OBJECT (obj), 0);
+ *
  *     return priv->foo;
  *   }
  *
@@ -1832,6 +1834,8 @@ guint     g_type_get_type_registration_serial (void);
  *   {
  *     MyObjectPrivate *priv = my_object_get_instance_private (obj);
  *
+ *     g_return_if_fail (MY_IS_OBJECT (obj));
+ *
  *     if (priv->bar != bar)
  *       priv->bar = bar;
  *   }
@@ -1843,6 +1847,10 @@ guint     g_type_get_type_registration_serial (void);
  * Also note that private structs added with these macros must have a struct
  * name of the form `TypeNamePrivate`.
  *
+ * It is safe to call _get_instance_private on %NULL or invalid object since
+ * it's only adding an offset to the instance pointer. In that case the returned
+ * pointer must not be dereferenced.
+ *
  * Since: 2.38
  */
 #define G_ADD_PRIVATE(TypeName) { \