Fix CID 355779: dynbuf cannot be NULL

After an earlier code cleanup, dns_rdatalist_tordataset() always succeeds, so the RETERR error handling macro below the function call was removed. After that change the dynbuf variable can never be NULL in the error handling code path under the failure label.

    *** CID 355779:  Null pointer dereferences  (REVERSE_INULL)
    /lib/dns/tkey.c: 997 in buildquery()
    991     		dns_message_puttempname(msg, &aname);
    992     	}
    993     	if (question != NULL) {
    994     		dns_rdataset_disassociate(question);
    995     		dns_message_puttemprdataset(msg, &question);
    996     	}
    >>>     CID 355779:  Null pointer dereferences  (REVERSE_INULL)
    >>>     Null-checking "dynbuf" suggests that it may be null, but ithas already been dereferenced on all paths leading to the check.
    997     	if (dynbuf != NULL) {
    998     		isc_buffer_free(&dynbuf);
    999     	}
    1000     	return (result);
    1001     }
    1002

Closes #3489 (closed)