dirmngr: Fix OCSP check.

* dirmngr/ocsp.c (ocsp_isvalid): Fix the condition, it's wrong
to require *TMP_TIME is zero.

Here, NEXT_UPDATE is valid (that is, next_update[0] is not zero).
So is TMP_TIME.

• Reported-by: Joshua Rogers <joshua@joshua.hu>
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>