VersionReq may panic .Found by afl.rs

[StevenJiang1110]

I've using afl.rs to fuzz this crate. And I found a panic issue in VersionReq. The code to replay the issue is

let _ = semver::VersionReq::parse("80000000000000000000");

And from the panic reportn, it seems that this panic is caused by use unwrap on a Result which may return err.

I hope you can check if this is a bug need to be fixed. Thanks a lot.

[dtolnay]

Thanks — this is fixed in 1.0.0 and I've put fuzz target for parsing into the repo.