weston-launch: quit if -t without -u

setup_tty() function uses the tty argument for choosing the tty/VT only
if wl->new_user (the -u option) is given. If the tty option is given
without -u, it will only be used for misleading error messages.

To make it clear to the user that -t without -u does not work the way
one might think, let weston-launch exit with an error in that case.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Acked-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>

libweston/weston-launch.c

@@ -722,6 +722,9 @@ main(int argc, char *argv[])
 	if ((argc - optind) > (MAX_ARGV_SIZE - 6))
 		error(1, E2BIG, "Too many arguments to pass to weston");
 
+	if (tty && !wl.new_user)
+		error(1, 0, "-t/--tty option requires -u/--user option as well");
+
 	if (wl.new_user)
 		wl.pw = getpwnam(wl.new_user);
 	else