Skip detach when delv's client never created

Skip detach when delv's client never created

run_resolve allocates dns_client_t late, but the cleanup epilogue
called dns_client_detach() unconditionally. When convert_name() or
dns_client_create() failed first, the detach hit a NULL client and
the REQUIRE(DNS_CLIENT_VALID) inside it aborted the process with
SIGABRT instead of a clean error exit.

Guard the detach with a NULL check. Add a digdelv test that runs
delv on a query name whose first label exceeds 63 octets and
asserts the process does not exit 134.

Assisted-by: Claude:claude-opus-4-7
(cherry picked from commit dba102ef16821f91f0494f9db5c9a646dcab913f)