approxidate: fix NULL dereference in date_time()

approxidate: fix NULL dereference in date_time()

When we see a time like "noon", we pass "12" to our date_time() helper,
which sets the hour to 12pm. If the current time is before noon, then we
wrap around to yesterday using date_yesterday(). But unlike the normal
calls to date_yesterday() from approxidate_alpha(), we pass a NULL "num"
parameter. Since c27cc94fad (approxidate: handle pending number for
"specials", 2018-11-02), that causes a segfault.

One way to fix this is by checking for NULL. But arguably date_time() is
abusing our helper by passing NULL in the first place (and this is the
only case where one of these "special" parsers is used this way). So
instead, let's have it just do the 1-day subtraction itself. It's still
just a one-liner due to our update_tm() helper.

Note that the test added here is a little funny, as we say "10am noon",
which makes the "10am" seem pointless.  But this bug can only be
triggered when it the currently-parsed hour is before the special time.
The latest special time is "tea" at 1700, but t0006 uses a hard-coded
TEST_DATE_NOW of 1900. We could reset TEST_DATE_NOW, but that may lead
to confusion in other tests. Just saying "10am noon" makes this test
self-contained.

Reported-by: Carlo Arenas <carenas@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>