TAT sending failures are not always handled properly

Code introduced by #392 (closed)/!484 (merged) improperly handles dns_view_zonecut() failures: even if that function returns something else than ISC_R_SUCCESS, the dns_rdataset_t structure passed to it may still be associated (e.g. with a covering NSEC record when DNS_R_NXDOMAIN is returned) . Not handling this properly may cause a reference leak, resulting in hangs upon shutdown.