Avoid memory corruption when display panel is disposed just after initialization

In cc_object_storage_create_dbus_proxy_sync and cc_object_storage_create_dbus_proxy_finish we need to use g_steal_pointer to make sure local_error is not double freed. Also dialog_got_proxy_cb and dialog_got_proxy_props_cb may be called after the instance of CcNightLightDialog has been disposed. Make sure 'self' pointer is not dereferenced if not valid.

Fixes: #86 (closed)