manager: guard the flag setting by the actual device presence

The parent might be an active connection w/o the device being determined.

src/nm-manager.c

@@ -3126,8 +3126,10 @@ _internal_activate_device (NMManager *self, NMActiveConnection *active, GError *
 		 * decides to go unmanaged while we're activating (perhaps because other slaves
 		 * go away leaving him with no kids).
 		 */
+		if (master_device) {
 			nm_device_set_unmanaged_by_flags (master_device, NM_UNMANAGED_EXTERNAL_DOWN,
 			                                  NM_UNMAN_FLAG_OP_FORGET, NM_DEVICE_STATE_REASON_USER_REQUESTED);
+		}
 
 		nm_active_connection_set_master (active, master_ac);
 		_LOGD (LOGD_CORE, "Activation of '%s' depends on active connection %p %s",