…5943

Since normal file descriptors and sockets cannot be mixed on the
platform.

For example, the following code was crashing.

```c
#include <mruby.h>
#include <mruby/compile.h>
#include <mruby/proc.h>

int
main(int argc, char *argv[])
{
  mrb_state *mrb = mrb_open();

  mrb_value fiber = mrb_load_string(mrb, "Fiber.new { Fiber.new { 12345.6789 }.transfer }");
  mrb_value res = mrb_fiber_resume(mrb, fiber, 0, NULL);
  mrb_p(mrb, res);

  mrb_close(mrb);

  return 0;
}
```