merge revision(s) 611e7110,a9c5c2d6: [Backport #17527]

	Test incorrect behaviour of `rb_io_wait_readable/writable`.

	---
	 test/fiber/test_io.rb | 35 +++++++++++++++++++++++++++++++++++
	 1 file changed, 35 insertions(+)

	Check errno before invoking scheduler in
	 `rb_io_wait_readable/writable`.

	See <https://bugs.ruby-lang.org/issues/17527> for more details.
	---
	 io.c | 66 +++++++++++++++++++++++++++++++++++-------------------------------
	 1 file changed, 35 insertions(+), 31 deletions(-)