fix at-exit close handling for non-main place
fix at-exit close handling for non-main place

This bug has been causing problems since the change to the `math`
library to register mpfr_free_cache() only once per place.

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