fuzz: add regression test for AST roundtripping

I couldn't get this to reproduce. Maybe some of my recent changes to
regex-syntax fixed this? Not sure.

I'm not a huge fan of this fuzzer in general because it isn't really
testing a rock solid guarantee that we provide. And the positions are
tough to deal with.

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=62382

Author: BurntSushi