[krb5-auth-dialog] Don't try to destroy context menu if it's not initialized

[Guido Günther <guidog src gnome org>]

commit 9c14e9701e61e4a78315e25e6dbd1b3809bf53f5
Author: Guido Günther <agx sigxcpu org>
Date:   Fri Jul 7 09:25:13 2017 +0200

    Don't try to destroy context menu if it's not initialized

 src/ka-applet.c |    6 ++++--
 1 files changed, 4 insertions(+), 2 deletions(-)
---
diff --git a/src/ka-applet.c b/src/ka-applet.c
index fb52f43..440a29f 100644
--- a/src/ka-applet.c
+++ b/src/ka-applet.c
@@ -1195,8 +1195,10 @@ ka_applet_destroy (KaApplet* self)
     gtk_widget_destroy (GTK_WIDGET(self->priv->prefs));
     self->priv->prefs = NULL;
 
-    gtk_widget_destroy (GTK_WIDGET(self->priv->context_menu));
-    self->priv->context_menu = NULL;
+    if (self->priv->context_menu) {
+        gtk_widget_destroy (GTK_WIDGET(self->priv->context_menu));
+        self->priv->context_menu = NULL;
+    }
 
     ka_kerberos_destroy ();
 }