last: fix wtmp user name buffer overflow [asan]

Ensure utmp user name field is null terminated.  Without that getpwnam() can
buffer overflow, when wtmp file is malformed.

Addresses: https://github.com/karelzak/util-linux/issues/715
Signed-off-by: Sami Kerola <kerolasa@iki.fi>

login-utils/last.c

@@ -600,6 +600,7 @@ static int is_phantom(const struct last_control *ctl, struct utmpx *ut)
 
 	if (ut->ut_tv.tv_sec < ctl->boot_time.tv_sec)
 		return 1;
+	ut->ut_user[__UT_NAMESIZE - 1] = '\0';
 	pw = getpwnam(ut->ut_user);
 	if (!pw)
 		return 1;