[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.

1 files changed, 2 insertions, 0 deletions

diff --git a/string.c b/string.c
index 6fd165cd5a..fc5629638c 100644
--- a/string.c
+++ b/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);*/