windows: improved handling of invalid FDs

If passed and invalid FD, _get_osfhandle() sets an error code
through errno, not _doserrno. Hence we need to use
SET_REQ_WIN32_ERROR insted of SET_REQ_RESULT.

In debug builds, _get_osfhandle() also raises a superfluous
assert. I implemented a wrapper that disables all asserts
around the call to _get_osfhandle().

This fixes node.js unit tests test-fs-read-stream.js and
test-listen-fd-ebadf.js.