New function: g_clear_object()

By analogy to g_clear_error, takes a pass-by-reference GObject reference
and, if non-%NULL, unrefs it and sets it equal to %NULL.

Bug #620263.

gobject/tests/reference.c

@@ -0,0 +1,35 @@
+#include <glib-object.h>
+
+static void
+test_clear (void)
+{
+  GObject *o = NULL;
+  GObject *tmp;
+
+  g_clear_object (&o);
+  g_assert (o == NULL);
+
+  tmp = g_object_new (G_TYPE_OBJECT, NULL);
+  g_assert_cmpint (tmp->ref_count, ==, 1);
+  o = g_object_ref (tmp);
+  g_assert (o != NULL);
+
+  g_assert_cmpint (tmp->ref_count, ==, 2);
+  g_clear_object (&o);
+  g_assert_cmpint (tmp->ref_count, ==, 1);
+  g_assert (o == NULL);
+
+  g_object_unref (tmp);
+}
+
+int
+main (int argc, char **argv)
+{
+  g_test_init (&argc, &argv, NULL);
+
+  g_type_init ();
+
+  g_test_add_func ("/object/clear", test_clear);
+
+  return g_test_run ();
+}