gcoap: don't drop observer on re-register

With #9209 gCoAP got the ability to re-register and OBSERVE with a new
token, sadly the `observer` variable wasn't set in that fix, so a
re-registration actually led to the deletion of the observer (because it
is still `NULL` when the old registration is overwritten in l. 317)

sys/net/application_layer/gcoap/gcoap.c

@@ -291,6 +291,7 @@ static size_t _handle_req(coap_pkt_t *pdu, uint8_t *buf, size_t len,
                     && _endpoints_equal(remote, resource_memo->observer)) {
                 /* observer re-registering with new token */
                 memo = resource_memo;
+                observer = resource_memo->observer;
             }
             else if ((empty_slot >= 0) && (resource_memo == NULL)) {
                 int obs_slot = _find_observer(&observer, remote);