[gnome-software] Fix a crash when clicking the back button

[Kalev Lember <klember src gnome org>]

commit facb84ebc865cb81a8711253f806f5c87c5421f7
Author: Kalev Lember <kalevlember gmail com>
Date:   Mon Jan 5 21:14:15 2015 +0100

    Fix a crash when clicking the back button
    
    Avoid crashing in the clicked handler in case we have an underflow in
    back_entry_stack. It's unclear how this can happen, but log a critical
    warning for now and ignore the underflow.
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1145782

 src/gs-shell.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
---
diff --git a/src/gs-shell.c b/src/gs-shell.c
index dc9a543..e5647e0 100644
--- a/src/gs-shell.c
+++ b/src/gs-shell.c
@@ -268,6 +268,8 @@ gs_shell_back_button_cb (GtkWidget *widget, GsShell *shell)
        GsShellPrivate *priv = shell->priv;
        BackEntry *entry;
 
+       g_return_if_fail (!g_queue_is_empty (priv->back_entry_stack));
+
        entry = g_queue_pop_head (priv->back_entry_stack);
 
        gs_shell_change_mode (shell, entry->mode, entry->app, entry->category, FALSE);