dns_ncache_current fails to set covered correctly

This was evidenced by the TYPE0 being reported in named_dump.db in the abbreviated output for RRSIG.

; pending-answer
                        374     \-DS    ;-$NXRRSET
; CK0POJMG874LJREF7EFN8430QVIT8BSM.com. NSEC3 1 1 0 - CK0Q2D6NI4I7EQH8NA30NS61O48UL8G5 NS SOA RRSIG DNSKEY NSEC3PARAM
; CK0POJMG874LJREF7EFN8430QVIT8BSM.com. RRSIG TYPE0 ...
; com. SOA a.gtld-servers.net. nstld.verisign-grs.com. 1694561028 1800 900 604800 86400
; com. RRSIG TYPE0 ...
; ND80U3J0AN2RDPNKU93HLF3KAEOQLPG2.com. NSEC3 1 1 0 - ND81C4V19VGF9H5HJNJT3IT3GFJVV9KP NS DS RRSIG
; ND80U3J0AN2RDPNKU93HLF3KAEOQLPG2.com. RRSIG TYPE0 ...