[Bug #19319] Fix crash in rb_str_casemap

	The following code crashes on my machine:

	```
	GC.stress = true

	str = "testing testing testing"

	puts str.capitalize
	```

	We need to ensure that the object `buffer_anchor` remains on the stack
	so it does not get GC'd.
	---
	 string.c | 2 ++
	 1 file changed, 2 insertions(+)