mruby/mruby — pull request #1763 by cubicdaiya (2014-02-27 17:12:54+00:00)
Title: fix SEGV bug for mrb_str_new_static

`mrb_str_new_static` has the potential to cause seg-fault when 3rd argument is negative.

```c
/* example for reproducing */
mrb_value s = mrb_str_new_static(mrb, "string", -1);
mrb_str_cat_lit(mrb, s, "hoge");
```