Update MLE receiver in Child ID req timeout error (#1634)

When Child ID request times out and there is no route to leader, attaching is
restarted. MLE receiver handler must be updated to avoid released
thread_attach_scanned_parent pointer usage if Child ID response is received
after this.

Author: Tero Heinonen

source/6LoWPAN/Thread/thread_host_bootstrap.c

@@ -1037,9 +1037,10 @@ static bool thread_child_id_req_timeout(int8_t interface_id, uint16_t msgId, boo
     cur->thread_info->releaseRouterId = false;
 
     cur->nwk_bootstrap_state = ER_BOOTSRAP_DONE;
-    mle_service_interface_receiver_handler_update(cur->id, thread_general_mle_receive_cb);
 
 exit:
+    mle_service_interface_receiver_handler_update(cur->id, thread_general_mle_receive_cb);
+
     ns_dyn_mem_free(cur->thread_info->thread_attach_scanned_parent);
     cur->thread_info->thread_attach_scanned_parent = NULL;