[seahorse] Fix right click signal callback

[Niels De Graef <nielsdg src gnome org>]

commit 9cd617828d990f022132a51f285a4daa8421a9fe
Author: Anukul Sangwan <anukulsangwan icloud com>
Date:   Wed Aug 22 19:36:03 2018 +0530

    Fix right click signal callback

 src/key-manager.vala | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)
---
diff --git a/src/key-manager.vala b/src/key-manager.vala
index d1ee558a..2164cc17 100644
--- a/src/key-manager.vala
+++ b/src/key-manager.vala
@@ -187,8 +187,13 @@ public class Seahorse.KeyManager : Catalog {
     }
 
     private bool on_keymanager_key_list_button_pressed(Gdk.EventButton event) {
-        if (event.button == 3)
+        if (event.button == 3) {
             show_context_menu(Catalog.MENU_OBJECT, event);
+            GLib.List<GLib.Object> objects = get_selected_objects();
+            if (objects.length() > 1) {
+                return true;
+            }
+        }
 
         return false;
     }