Check for invalid leading zeros in CBS_get_asn1_uint64.

Check for invalid leading zeros in CBS_get_asn1_uint64.

ASN.1 integers cannot have all zeros or all ones for the first 9 bits.
This rule ensures the numbers are encoded with the smallest number of
content octets (see ITU-T Rec X.690 section 8.3.2).

Based on BoringSSL commit 5933723b7b592e9914f703d630b596e140c93e16

ok deraadt@ jsing@