[gtk+/portal] Don't call quit if the loop is NULL

[Matthias Clasen <matthiasc src gnome org>]

commit 29e76ad0d51c4c975ef5ad3ec1afaee665c4ec42
Author: Matthias Clasen <mclasen redhat com>
Date:   Wed Jul 6 22:48:37 2016 -0400

    Don't call quit if the loop is NULL
    
    As it will be, in the async case.

 gtk/gtkprintoperation-portal.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/gtk/gtkprintoperation-portal.c b/gtk/gtkprintoperation-portal.c
index 93a77f5..70fb1a1 100644
--- a/gtk/gtkprintoperation-portal.c
+++ b/gtk/gtkprintoperation-portal.c
@@ -433,7 +433,8 @@ prepare_print_called (GObject      *source,
     {
       g_warning ("Error: %s", error->message);
       g_error_free (error);
-      g_main_loop_quit (portal->loop);
+      if (portal->loop)
+        g_main_loop_quit (portal->loop);
       return;
     }
   else