Rewrite and fix X509v3_asid_subset()

Rewrite and fix X509v3_asid_subset()

X509v3_asid_subset() assumes that both asnum and rdi are present while
they are both marked OPTIONAL in RFC 3779, 3.2.3. It will crash if
either one is missing. In RPKI land RDI is a MUST NOT use (e.g, RFC
6487, 4.8.11), so this API is currently useless (and seemingly unused).

Pick apart an ugly logical pipeline and implement this check in a
readable fashion.

ok jsing