vt: move vc_saved_screen to within tty allocated judgment

vt: move vc_saved_screen to within tty allocated judgment

Everything starts with the assumption that a tty has been allocated.
Therefore, Move it to within the tty allocation check.

Fixes: 23743ba64709 ("vt: add support for smput/rmput escape codes")
Reported-by: syzbot+f6cb41c144427dc0796a@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=f6cb41c144427dc0796a
Tested-by: syzbot+f6cb41c144427dc0796a@syzkaller.appspotmail.com
Signed-off-by: Edward Adam Davis <eadavis@qq.com>
Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/tencent_CAD45DB31906CF890DBB25AB0DED12205D07@qq.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>