[libgepub] Add guard to _set_paginate()

[Daniel Garcia Moreno <danigm src gnome org>]

commit c1fc28a9b85ede0b68f322e8dd6166717fd7e79b
Author: Bastien Nocera <hadess hadess net>
Date:   Wed May 31 15:02:12 2017 +0200

    Add guard to _set_paginate()
    
    https://bugzilla.gnome.org/show_bug.cgi?id=768002

 libgepub/gepub-widget.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
---
diff --git a/libgepub/gepub-widget.c b/libgepub/gepub-widget.c
index 0b78272..b5d3a85 100644
--- a/libgepub/gepub-widget.c
+++ b/libgepub/gepub-widget.c
@@ -503,6 +503,8 @@ void
 gepub_widget_set_paginate (GepubWidget *widget,
                            gboolean p)
 {
+    g_return_if_fail (GEPUB_IS_WIDGET (widget));
+
     widget->paginate = p;
     reload_current_chapter (widget);
 }