Somewhat-related changes to expr.c & tree.c

[Richard Kenner]

    If you allocate enough contiguous storage following a structure
    ending in a zero-length array, you're allowed to dereference it.
    This has been a GCC extension for ages, and one that C99 picked up.

I was wondering if that might have been the issue, but it wasn't clear
from your message that it was.

Hmmm.  I have no good suggestions here.  In C, sizetype is unsigned, but
the upper bound is -1, which overflows.

This clearly needs more thought.