[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

@@ -7329,6 +7329,8 @@ rb_str_casemap(VALUE source, OnigCaseFoldType *flags, rb_encoding *enc)
     DATA_PTR(buffer_anchor) = 0;
     mapping_buffer_free(current_buffer);
 
+    RB_GC_GUARD(buffer_anchor);
+
     /* TODO: check about string terminator character */
     str_enc_copy(target, source);
     /*ENC_CODERANGE_SET(mapped, cr);*/