Fix time offset calculation.

ASN1_GENERALIZEDTIME and ASN1_UTCTIME may be specified using offsets,
even though that's not supported within certificates.

To convert the offset time back to GMT, the offsets are supposed to be
subtracted, not added. e.g. 1759-0500 == 2359+0100 == 2259Z.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2654)