Fix ASN1_INTEGER handling.

Only treat an ASN1_ANY type as an integer if it has the V_ASN1_INTEGER
tag: V_ASN1_NEG_INTEGER is an internal only value which is never used
for on the wire encoding.

Thanks to David Benjamin <davidben@google.com> for reporting this bug.

This was found using libFuzzer.

RT#4364 (part)CVE-2016-2108.

Reviewed-by: Emilia Käsper <emilia@openssl.org>