Validate protocol regexp object earlier

One of the reasons PR 93 (https://github.com/denoland/rust-urlpattern/pull/93) was needed is that `compute_protocol_matches_special_scheme` [doesn't check](https://github.com/denoland/rust-urlpattern/blob/main/src/constructor_parser.rs#L261-L265) for an error-free regexp object, as done with other component compilations that check `optionally_transpose_regex_error`, for [example](https://github.com/denoland/rust-urlpattern/blob/main/src/lib.rs#L352).

PR 93 prevents the crash, and the error will bubble up the same, but I think we can still catch this before getting as far as matching on the protocol (even though this is kind of a special case of needing to parse and match on the protocol so the parser can create an Init object correctly).