Check for trailing garbage in X509_addr_get_afi()

Check for trailing garbage in X509_addr_get_afi()

Per RFC 3779 2.2.3.3, the addressFamily field contains the 2-byte AFI
and an optional 1-byte SAFI. Nothing else. The optional SAFI is nowhere
exposed in the API. It is used expliclty only for pretty printing. There
are implicit uses in a few places, notably for sorting/comparing where
trailing garbage would be erroneously taken into account.

Erroring in this situation will let us avoid this in upcoming revisions.

ok inoguchi jsing