common: Guarantee that gnupg_get_time does not return an error.

* common/gettime.c (gnupg_get_time): Abor if time() failed.
(gnupg_get_isotime): Remove now useless check.
(make_timestamp): Remove check becuase we already checked this modulo
the faked time thing.

In reality a call foo = time (NULL) can never fail because the only
defined error is EFAULT, but we don't provide a buffer.

• Signed-off-by: Werner Koch <wk@gnupg.org>