Use an unsigned type for sizes.
Use an unsigned type for sizes.

Fixes this crash http://drdr.racket-lang.org/32121/pkgs/racket-test/tests/racket/stress/fuzz.rkt
found by fuzz testing.

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