handle NIL in LSUB in addition to LIST too.

Whoops!  I was hurrying a bit and although I dealt with LIST in https://github.com/whiteout-io/browserbox/pull/27 I completely forgot to check and handle the LSUB case in my haste.  (We hadn't previously used LSUB ourselves.)  Apologies; I'll try and make sure to run full local tests before putting up pull requests in the future.  Things should get easier when we're 100% up-to-date with browserbox upstream in our rep.