Checking whether a EDDSA key was private or not was broken could lead to
attempting to sign records with a public key and this could cause a
segmentation failure (read of a NULL pointer) within OpenSSL.
Closes #4855
Merge branch '4855-openssleddsa_isprivate-needs-to-supply-a-buffer' into 'bind-9.18'
See merge request isc-projects/bind9!9329