fix another potential overflow in fixnum `expt`
fix another potential overflow in fixnum `expt`

This commit is a follow-up to e96d592735. The bug fixed this time is
more subtle, because the overflowing computation is never used, but
that doesn't matter in terms of avoiding undefined behavior.

Thanks for Pascal Cuoq and John Regehr.

mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket