string.c: `mrb_str_to_cstr()` should always return a mutable string.

src/string.c

@@ -835,8 +835,15 @@ mrb_str_to_cstr(mrb_state *mrb, mrb_value str0)
 {
   struct RString *s;
 
+  const char *p = RSTRING_PTR(str0);
+  size_t len = RSTRING_LEN(str0);
   check_null_byte(mrb, str0);
-  s = str_new(mrb, RSTRING_PTR(str0), RSTRING_LEN(str0));
+  if (RSTR_EMBEDDABLE_P(len)) {
+    s = str_init_embed(mrb_obj_alloc_string(mrb), p, len);
+  }
+  else {
+    s = str_init_normal(mrb, mrb_obj_alloc_string(mrb), p, len);
+  }
   return RSTR_PTR(s);
 }