[gnome-control-center] Do not abort if an unknown user is signaled removed

[Frederic Peters <fpeters src gnome org>]

commit f1d70389c0e5dc6e2c62c4f5510ff75b62b73ebf
Author: Frédéric Péters <fpeters 0d be>
Date:   Fri Mar 18 11:58:32 2011 +0100

    Do not abort if an unknown user is signaled removed
    
    https://bugzilla.gnome.org/show_bug.cgi?id=645118

 panels/user-accounts/um-user-manager.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
---
diff --git a/panels/user-accounts/um-user-manager.c b/panels/user-accounts/um-user-manager.c
index 1e39018..6b603bb 100644
--- a/panels/user-accounts/um-user-manager.c
+++ b/panels/user-accounts/um-user-manager.c
@@ -226,6 +226,8 @@ user_deleted_handler (DBusGProxy *proxy,
         UmUser *user;
 
         user = g_hash_table_lookup (manager->user_by_object_path, object_path);
+        if (!user)
+                return;
         g_object_ref (user);
         g_signal_handlers_disconnect_by_func (user, user_changed_handler, manager);