[Bug #20755] Frozen string should not be writable via IO::Buffer

	[Bug #20752] Slice of readonly `IO::Buffer` also should be readonly

* Fix `io_buffer_get_string` default length computation.

When an offset bigger than the size is given, the resulting length will be
computed incorrectly. Raise an argument error in this case.

* Validate all arguments.

- Fix IO::Buffer `read`/`write` to use a minimum length.

	[Bug #19754] Make `IO::Buffer#get_string` check `offset` range
	 (#8016)

	---
	 io_buffer.c                 | 3 +++
	 test/ruby/test_io_buffer.rb | 8 ++++++++
	 2 files changed, 11 insertions(+)

	Fix mutation on shared strings. (#7837)

	---
	 io_buffer.c                 | 19 ++++++++++++-------
	 test/ruby/test_io_buffer.rb |  4 ----
	 2 files changed, 12 insertions(+), 11 deletions(-)

	Fix incorrect usage of `rb_fiber_scheduler_io_(p)(read|write)`.
	 (#7593)

	---
	 io_buffer.c                  |  8 ++++----
	 scheduler.c                  | 48 ++++++++++++++++++++++----------------------
	 test/fiber/test_io_buffer.rb | 33 ++++++++++++++++++++++++++++++
	 3 files changed, 61 insertions(+), 28 deletions(-)