I have no idea how long this bug has existed for.  I
could well be a long time.  I noticed it while trying
to kill off SET_LINENO.

To reproduce, build a debug python, cd to Tools/freeze/
and run

$ /path/to/python -O freeze.py hello.py

I get a wodge ot modulefinder (I presume) output, and then:

freezing BaseHTTPServer ...
freezing FixTk ...
freezing SocketServer ...
freezing StringIO ...
freezing Tkconstants ...
freezing Tkinter ...
python: ../Python/marshal.c:66: w_more: Assertion
`(int)(char)(c) == (c)' failed.
Aborted (core dumped)

This is on x86; on my PPC laptop where I discovered
this, it gets quite a bit further before dying.  So
maybe some endianness fun.

The assert that fails is in a Py_SAFEDOWNCAST macro.

I have *no* idea where to start with this one;
assigning to Thomas because I have a vague recollection
he knows something about how freeze works <evil cackle>
-- feel free to reassign!