boost::algorithm::split_regex hangs and consumes CPU on dangling alternatives

I happened to typo a regex, and I noticed that `boost::algorithm::split_regex` started consuming a bunch of CPU. I don't know is this would have eventually caught itself and terminating. The regex in question was `;|`, and this appears to be true for any dangling `|`.

I'm a bit surprised that `boost::regex` allowed `;|` as valid, given split_regex's behavior.

Simple example up at https://gist.github.com/directionless/c38eb7dc216bcd93b40c3ecaf99f326e

I tested this on darwin with boost 1.67, and on linux with boost 1.65

(This was moved here from https://github.com/boostorg/regex/issues/76)