Address a few type-checking warnings
diff --git a/glib/tests/atomic.c b/glib/tests/atomic.c
index 5926e31..9b4d800 100644
--- a/glib/tests/atomic.c
+++ b/glib/tests/atomic.c
@@ -83,14 +83,14 @@
   res = g_atomic_pointer_compare_and_exchange (&vp, &s, &s);
   g_assert_false (res);
   g_assert_true (vp == 0);
-  res = g_atomic_pointer_compare_and_exchange (&vp, 0, 0);
+  res = g_atomic_pointer_compare_and_exchange (&vp, NULL, NULL);
   g_assert_true (res);
   g_assert_true (vp == 0);
 
   g_atomic_pointer_set (&ip, 0);
   ip2 = g_atomic_pointer_get (&ip);
   g_assert_true (ip2 == 0);
-  res = g_atomic_pointer_compare_and_exchange (&ip, 0, 0);
+  res = g_atomic_pointer_compare_and_exchange (&ip, NULL, NULL);
   g_assert_true (res);
   g_assert_true (ip == 0);
 
@@ -98,7 +98,7 @@
   vp2 = (gpointer) g_atomic_pointer_get (&gs);
   gs2 = (gsize) vp2;
   g_assert_cmpuint (gs2, ==, 0);
-  res = g_atomic_pointer_compare_and_exchange (&gs, 0, 0);
+  res = g_atomic_pointer_compare_and_exchange (&gs, NULL, NULL);
   g_assert_true (res);
   g_assert_cmpuint (gs, ==, 0);
   gs2 = (gsize) g_atomic_pointer_add (&gs, 5);