Check the file descriptor with IO#initialize; resolve #4966

[dearblue]

No description provided.

[matz]

The issue was fixed by 7add524. If there's any uncovered issue, tell me.

[dearblue]

I don't think there is a problem with select.
# I wasn't aware of the FD_SETSIZE (I'm embarrassed).

However, it' s worthwhile to check file descriptors with IO#initialize:

• CRuby removes bad file descriptors with IO#initialize.
  https://github.com/ruby/ruby/blob/v2_7_1/io.c#L8270-L8275

• On Windows, if you give an invalid file descriptor to _get_osfhandle(), it seems to crash.
  This will cause a crash if you give it to a POSIX compatible file function (such as fstat(), read() and write()).

  It couldn't be replicated in wine, but I'm guessing from the failure in appveyor.
  This is cluttered with commit references in Segmentation fault at mrb_io_s_select #4966 by CI failures (I'm embarrassed this too), most of which occurred when I give _get_osfhandle() a non-existent file descriptor.

[matz]

Ah, according to POSIX, all those system calls should fail gracefully with EBADF. I didn't know _get_osfhandle() crashes on Windows (sigh).

OK, I will merge this PR with a slight modification.