Fix warning of type mismatch
Fix warning of type mismatch

```
../src/onig-string.cc:30:40: warning: comparison between signed and
unsigned integer expressions [-Wsign-compare]
   hasMultiByteChars = (value->Length() != utf8_length_);
```

alexandrudima/node-oniguruma,atom/node-oniguruma,atom/node-oniguruma,atom/node-oniguruma,alexandrudima/node-oniguruma,alexandrudima/node-oniguruma