[racket] FFI - pointer to an array of structs

Well, the native function doesn't need a size, becaue it is
responsible for an allocation - that's why it needs a pointer to a
pointer. As I understand i only need to allocate memory for this
pointer and for one integer whose address is passęd as well, and this
is where native function writes the size of allocated array.
That's why I tried to use '_ptr o' for this.