winhttp: Prevent swallowing of url parsing error

This has been discovered while working on release LibGit2Sharp v0.19 (cf. libgit2/libgit2sharp#798).

Test `CloneFixture::CloningAnUrlWithoutPathThrows()` was passing on Mono, but failing on Windows, returning a generic error code instead of `GIT_EINVALIDREFSPEC`.

This fix the issue by propagating up to caller the parsing error.