[RFA] Fix PR 598 and PR 776

[Richard Kenner]

    > I don't understand this patch.  Where is the abort for
    > allocations variable-length temporaries now?

    The same place it was before - size is still -1 since they fail the
    
        TYPE_SIZE_UNIT (TYPE_MAIN_VARIANT (type)) != 0

    test.

I'm confused.  TYPE_SIZE_UNIT is certainly nonzero, it's just not a 
constant, but an expression.

With your change, it will produce a message that the type is too large,
when the issue is that it's of variable size.