fix: dev: Reject RSA DNSKEYs with degenerate modulus

A crafted DNSKEY rdata whose declared exponent length consumed the
whole buffer produced an RSA key with no modulus, which dnssec-importkey
accepted as valid and wrote to a .private file with no key material.
The wire-format parser now rejects RSA public keys with a modulus
smaller than 512 bits, the lowest legitimate size across the RSA
DNSSEC algorithms.

Closes #5920

Merge branch '5920-opensslrsa-fromdns-zero-modulus-accepted' into 'main'

See merge request isc-projects/bind9!11929