Trivial fix in value_sub

[Jim Blandy]

Jim Ingham <jingham@apple.com> writes:
> So... I don't think you should keep the size at 0.  This seems like
> gdb is just silently ignoring the " -  x" part of what they typed, and
> you should always be explicit about what you have done.  But if you
> think an error is more appropriate, I am fine with that...

Oh, no, I didn't mean to suggest that zero was the right size to use;
I agree with you completely that that would be pretty confusing.

Your story is pretty amazing --- I would never have guessed that
people actually *use* the sizeof (struct incomplete) == 1 behavior!  I
think it is much more common for people to be unaware that the type is
incomplete; if this hunch is correct, then the behavior your toolbox
folks love will be very confusing.  I think an error for arithmetic on
any incomplete type other than (void *) is the right thing.