merge revision(s) 2214bcb70d9f9120f1f3790ca340236c8f080991: [Backport #19792]

	Fix premature string collection during append

	Previously, the following crashed due to use-after-free
	with AArch64 Alpine Linux 3.18.3 (aarch64-linux-musl):

	```ruby
	str = 'a' * (32*1024*1024)
	p({z: str})
	```

	32 MiB is the default for `GC_MALLOC_LIMIT_MAX`, and the crash
	could be dodged by setting `RUBY_GC_MALLOC_LIMIT_MAX` to large values.
	Under a debugger, one can see the `str2` of rb_str_buf_append()
	getting prematurely collected while str_buf_cat4() allocates capacity.

	Add GC guards so the buffer of `str2` lives across the GC run
	initiated in str_buf_cat4().

	[Bug #19792]
	---
	 string.c | 2 ++
	 1 file changed, 2 insertions(+)

0 files changed, 0 insertions, 0 deletions