Prevents that OPENSSL_gmtime incorrectly signals success if gmtime_r fails,...

Prevents that OPENSSL_gmtime incorrectly signals success if gmtime_r fails, and that struct* tm result's possibly uninitialized content is used

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1613)