log: do not segfault on gmtime errors

log: do not segfault on gmtime errors

Many code paths assume that show_date and show_ident_date
cannot return NULL. For the most part, we handle missing or
corrupt timestamps by showing the epoch time t=0.

However, we might still return NULL if gmtime rejects the
time_t we feed it, resulting in a segfault. Let's catch this
case and just format t=0.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>