[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(-)

	IO::Buffer#resize: Free internal buffer if new size is zero (#7569)

	`#resize(0)` on an IO::Buffer with internal buffer allocated will
	result in calling `realloc(data->base, 0)`. The behavior of `realloc`
	with size = 0 is implementation-defined (glibc frees the object
	and returns NULL, while BSDs return an inaccessible object). And
	thus such usage is deprecated in standard C (upcoming C23 will make it
	UB).

	To avoid this problem, just `free`s the memory when the new size is zero.
	---
	 io_buffer.c                 |  5 +++++
	 test/ruby/test_io_buffer.rb | 18 ++++++++++++++++++
	 2 files changed, 23 insertions(+)

There is currently a bug ([#19084]) in how extended strings work in
IO Buffers. Object Shapes changes will make the string in this test
extended on 32 bit machines. Since this behavior is currently broken
(unrelated to object shapes) on 32 bit machines, this test will then
fail. We preemptively omit it so that this commit can be reverted
once the bug is fixed.