Prevent an infinite loop parsing a capture name

[stevenjohnstone]

Fixes #3729.

[Earlopain]

Thanks, can you add a test for this? In test/prism/fixtures create regex_capture.txt or similar that contains /(?<\u{21!3}>foo)/ =~ "foo" and /(?<\u{21!3}>foo)/. When you run tests with UPDATE_SNAPSHOTS=1 bundle exec rake, a snapshot file gets created that you should commit.

(I guess some other parsers we test against may correctly reject this code. If that is the case, simply excluding this new file for them is fine)

[Earlopain]

Sorry for the delay. Since this seems to be correctly implemented everywhere but prism, I just dropped the test. I now feel that wasn't such a bright idea after all.