Panic on some regex patterns

[Stranger6667]

On {"pattern": "\\u"}, schema compilation panics with:

thread 'main' panicked at 'index out of bounds: the len is 2 but the index is 2', /home/stranger6667/.cargo/registry/src/github.com-1ecc6299db9ec823/fancy-regex-0.6.0/src/parse.rs:394:17

Though, it seems like an issue with fancy-regex itself as it takes ix'th byte without checking the bounds.