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 <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

date.c

@@ -184,8 +184,10 @@ const char *show_date(unsigned long time, int tz, enum date_mode mode)
 		tz = local_tzoffset(time);
 
 	tm = time_to_tm(time, tz);
-	if (!tm)
-		return NULL;
+	if (!tm) {
+		tm = time_to_tm(0, 0);
+		tz = 0;
+	}
 
 	strbuf_reset(&timebuf);
 	if (mode == DATE_SHORT)