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

crypto/o_time.c

@@ -56,7 +56,8 @@ struct tm *OPENSSL_gmtime(const time_t *timer, struct tm *result)
      * should return &data, but doesn't on some systems, so we don't even
      * look at the return value
      */
-    gmtime_r(timer, result);
+    if (gmtime_r(timer, result) == NULL)
+        return NULL;
     ts = result;
 #elif !defined(OPENSSL_SYS_VMS) || defined(VMS_GMTIME_OK)
     ts = gmtime(timer);