Absref bugfix

Potential bug fix for tests/todo/absref-crash.hs

Still slightly queasy about it it -- it `pad`s missing `RProp` using `top` which is ok, I guess. But the question is WHY are those missing?

TODO: 
1. check if it breaks any tests (unlikely)
2. check if it actually suffices for `Repeat.hs` and `IcfpDemo.hs`

@gridaphobe: can you check the TODO items above? I can't run it as they require your tweaks to the parser.
Perhaps you can merge this with your `Repeat` branch and see if the tests succeed? If so we could merge...


@ranjitjhala we've got a bunch of failing neg-tests now, and I suspect they're due to this line. Seems to me we should return both lists as is if they have equal length.