[gtk/wip/chergert/quartz4u] macos: style cleanup

[Christian Hergert <chergert src gnome org>]

commit eb9f50cde1dc7564815385577f209685fa19f625
Author: Christian Hergert <chergert redhat com>
Date:   Mon May 4 12:15:48 2020 -0700

    macos: style cleanup

 gdk/macos/gdkmacosdisplay.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)
---
diff --git a/gdk/macos/gdkmacosdisplay.c b/gdk/macos/gdkmacosdisplay.c
index 04894726a1..d8cc9eff34 100644
--- a/gdk/macos/gdkmacosdisplay.c
+++ b/gdk/macos/gdkmacosdisplay.c
@@ -510,10 +510,12 @@ _gdk_macos_display_from_display_coords (GdkMacosDisplay *self,
                                         int             *out_x,
                                         int             *out_y)
 {
-  if (out_y)
+  g_return_if_fail (GDK_IS_MACOS_DISPLAY (self));
+
+  if (out_y != NULL)
     *out_y = self->height - y + self->min_y;
 
-  if (out_x)
+  if (out_x != NULL)
     *out_x = x - self->min_x;
 }