Fix a potential assertion failure on shutdown in ns__client_endrequest. Scenario:
- We are shutting down, interface->clientmgr is gone.
- We receive a packet, it gets through ns__client_request
- mgr == NULL, return
- isc_nmhandle_detach calls ns_client_reset_cb
- ns_client_reset_cb calls ns_client_endrequest
- INSIST(client->state == NS_CLIENTSTATE_WORKING ||
client->state == NS_CLIENTSTATE_RECURSING) is not met
- we haven't started processing this packet so client->state == NS_CLIENTSTATE_READY. As a solution - don't do anything in ns_client_reset_cb if the client is still in READY state.
(cherry picked from commit b0888ff0)
Closes #1623 (closed)