[gnome-photos] preview-nav-buttons: Be more strict about what is acceptable

[Debarshi Ray <debarshir src gnome org>]

commit 15817b8ec9638147e9f61b6461803f3c3f634c00
Author: Debarshi Ray <debarshir gnome org>
Date:   Thu Jun 1 15:19:16 2017 +0200

    preview-nav-buttons: Be more strict about what is acceptable

 src/photos-preview-nav-buttons.c |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)
---
diff --git a/src/photos-preview-nav-buttons.c b/src/photos-preview-nav-buttons.c
index ce8e62e..90bfbc2 100644
--- a/src/photos-preview-nav-buttons.c
+++ b/src/photos-preview-nav-buttons.c
@@ -607,6 +607,8 @@ photos_preview_nav_buttons_new (PhotosPreviewView *preview_view, GtkOverlay *ove
 void
 photos_preview_nav_buttons_hide (PhotosPreviewNavButtons *self)
 {
+  g_return_if_fail (PHOTOS_IS_PREVIEW_NAV_BUTTONS (self));
+
   self->visible = FALSE;
   self->visible_internal = FALSE;
   photos_preview_nav_buttons_update_visibility (self);
@@ -627,6 +629,8 @@ photos_preview_nav_buttons_set_mode (PhotosPreviewNavButtons *self, PhotosWindow
 void
 photos_preview_nav_buttons_show (PhotosPreviewNavButtons *self)
 {
+  g_return_if_fail (PHOTOS_IS_PREVIEW_NAV_BUTTONS (self));
+
   self->visible = TRUE;
   self->visible_internal = TRUE;
   photos_preview_nav_buttons_update_visibility (self);