[luatex] Crash: type(r) == unset_node

[Javier Bezos]

The following minimal file crashes with TL2016 (win7/64)
and TL2015:

\documentclass{article}

\directlua{
luatexbase.add_to_callback("hpack_filter", function() end, 'desc')
}

------------
Assertion failed!

Program: C:\...\lualatex.exe
File: ./tex/align.w
Line: 966

Expression: type(r) == unset_node
------------

Unfortunately, using directly callback.register() doesn't work
in LaTeX any more :-(, but it seems the problem is in luatexbase
because using the latter in plain passes.

Javier