adding mruby-string-utf8 breaks String#[float]

#2651 is not perfect because same code exists in mruby-string-utf8 too.  When we build mruby with mruby-string-utf8:

```
% bin/mruby -e 'p "abc"[1.1]'
nil
```

Without mruby-string-utf8, it returns "b" as expected.