Fix decoding of ASN.1 LONG and ZLONG items

LONG and ZLONG items (which are OpenSSL private special cases of
ASN1_INTEGER) are encoded into DER with padding if the leading octet
has the high bit set, where the padding can be 0x00 (for positive
numbers) or 0xff (for negative ones).

When decoding DER to LONG or ZLONG, the padding wasn't taken in
account at all, which means that if the encoded size with padding
is one byte more than the size of long, decoding fails.  This change
fixes that issue.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3000)
(cherry picked from commit ca2045dc)