ffi/unsafe: unbreak `_list-struct`
ffi/unsafe: unbreak `_list-struct`

Broken by 0b1f96ab3d (specifically one of my changes to
Tobias's patch).

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