Fix handling of TAT sending failures

dns_view_zonecut() may associate the dns_rdataset_t structure passed to
it even if it returns a result different then ISC_R_SUCCESS.  Not
handling this properly may cause a reference leak.  Fix by ensuring
'nameservers' is cleaned up in all relevant failure modes.
16 jobs for 423-fix-handling-of-tat-sending-failures in 8 minutes and 8 seconds (queued for 7 seconds)