List regression

Two more minor bugs.

I also tracked down the dense matrix valgrind issue, and it comes from not unregistering a variable when the ArgumentError is raised in the nmatrix spec "adequately requires information to access a single entry of a dense matrix".  There's no easy fix to this at the moment without adding some way to trap ruby exceptions in the C code.  (This also isn't a big deal, as all this is doing is passing an extra address to the marking function.)