logind: fix crash when shutdown is not issued from a tty

It's possible that sd_bus_creds_get_tty() fails and thus
scheduled_shutdown_tty is NULL in method_schedule_shutdown().

Fix logind_wall_tty_filter() to get along with that, by showing the message on
all TTYs, instead of crashing in strcmp().

https://launchpad.net/bugs/1553040

src/login/logind-utmp.c

@@ -65,7 +65,7 @@ bool logind_wall_tty_filter(const char *tty, void *userdata) {
 
         assert(m);
 
-        if (!startswith(tty, "/dev/"))
+        if (!startswith(tty, "/dev/") || !m->scheduled_shutdown_tty)
                 return true;
 
         return !streq(tty + 5, m->scheduled_shutdown_tty);