merge revision(s) c5cf4d4e: [Backport #19542]

	Improve behavioural consistency of unallocated (zero length)
	 `IO::Buffer`. (#9532)

	This makes the behaviour of IO::Buffer.new(0) and IO::Buffer.new.slice(0, 0) consistent.

	Fixes https://bugs.ruby-lang.org/issues/19542 and https://bugs.ruby-lang.org/issues/18805.
	---
	 io_buffer.c                 | 14 ++++++--------
	 test/ruby/test_io_buffer.rb | 35 +++++++++++++++++++++++++++++++++++
	 2 files changed, 41 insertions(+), 8 deletions(-)