[racket] Freeing FFI resources

I am dealing with a foreign library that has functions that return
error messages in a char** input argument. These need to be explicitly
freed by calling another function. I figured out how to get them into
racket strings by using the _ptr and the _string ctype. But I didn't
see a way to capture the value before conversion to a racket string so
that I could free the original string, and continue to use the _string
conversion process. Is there an easy way to do these two thing
together?

-Eric