Don't issue ns_client_endrequest on a NS_CLIENTSTATE_READY client.

Don't issue ns_client_endrequest on a NS_CLIENTSTATE_READY client.

Fix a potential assertion failure on shutdown in ns__client_endrequest.
Scenario:
1. We are shutting down, interface->clientmgr is gone.
2. We receive a packet, it gets through ns__client_request
3. mgr == NULL, return
4. isc_nmhandle_detach calls ns_client_reset_cb
5. ns_client_reset_cb calls ns_client_endrequest
6. 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 b0888ff0398687793c83a46a7ce64eea1ca24899)