date: recognize bogus FreeBSD gmtime output

Most gmtime implementations return a NULL value when they
encounter an error (and this behavior is specified by ANSI C
and POSIX).  FreeBSD's implementation, however, will simply
leave the "struct tm" untouched.  Let's also recognize this
and convert it to a NULL (with this patch, t4212 should pass
on FreeBSD).

Reported-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
t4212: loosen far-in-future test for AIX

One of the tests in t4212 checks our behavior when we feed
gmtime a date so far in the future that it gives up and
returns NULL. Some implementations, like AIX, may actually
just provide us a bogus result instead.

It's not worth it for us to come up with heuristics that
guess whether the return value is sensible or not. On good
platforms where gmtime reports the problem to us with NULL,
we will print the epoch value. On bad platforms, we will
print garbage.  But our test should be written for the
lowest common denominator so that it passes everywhere.

Reported-by: Charles Bailey <cbailey32@bloomberg.net>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>