rpki-client: fix seqnum upper bound

rpki-client: fix seqnum upper bound

An ASN1_INTEGER doesn't contain the ASN.1 encoding in its data. It contains
a BIGNUM. In particular, there's no padding octet for integers with the top
bit of the top octet set. Do the check the dumb way and check all the parts
individually: non-negative, not larger than 20 octets and bit 159 not set.

Reported by Tom Harrison

ok claudio job