initialize the whole date structure.

Reference: /n/atom/patch/applied2013/expireuninit
Date: Fri Aug 23 05:36:08 CES 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/auth/lib/getexpiration.c	Fri Aug 23 05:35:45 2013
+++ /sys/src/cmd/auth/lib/getexpiration.c	Fri Aug 23 05:35:46 2013
@@ -13,8 +13,7 @@
 	Tm date;
 	int i;
 
-	date.year = date.mon = date.mday = 0;
-	date.hour = date.min = date.sec = 0;
+	memset(&data, 0, sizeof date);
 	for(i = 0; i < 8; i++)
 		if(!isdigit(d[i]))
 			return date;