Selectors not understood by the PathParser make PathParser.parse hang on Linux.

To reproduce:

$ docker run --rm -it swift
$ git clone https://github.com/cburgmer/SwiftPath.git
$ cd SwiftPath/
$ git checkout hang-on-linux
$ swift test --enable-test-discovery
... crickets ...

Judging by CPU usage it looks like an endless loop somewhere inside.

Expected

For running https://github.com/cburgmer/json-path-comparison I need to be able to execute an implementation in Ubuntu via Docker. This issue needs to be solved before including SwiftPath in the comparison.