[9.18] fix: nil: Release gnamebuf also on the error path

In dst_gssapi_acceptctx(), the gnamebuf could leak a little bit of
memory if dns_name_fromtext() would theoretically fail.  This would
require a Kerberos principal with invalid DNS name.

Closes #5737

Backport of MR !11536

Merge branch 'backport-5737-memory-leak-in-dst_gssapi_acceptctx-on-dns_name_fromtext-failure-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!11538