Fix dns_client_addtrustedkey(dns_rdatatype_dnskey)
Use a buffer that is big enough for DNSKEY records as well as DS records.
This fixes a problem with delv reported on the bind-users list https://lists.isc.org/pipermail/bind-users/2020-February/102714.html
Closes: #1647
Edited by Ondřej Surý