fix: don't report issue related to invalid position

[ldez]

Fixes #21

[kkHAIKE]

I really want to understand what kind of problem this is because in content check, each issue is quite important; otherwise, they cannot be connected.

Also, I hope this fix preferably doesn't alter the condition logic; just skipping Reportf will suffice.

func (r *runner) Reportf(pos token.Pos, format string, args ...interface{}) {
    if pos..IsValid() {
        r.pass.Reportf(pos, format, args...)
    }
}

[ldez]

I improved the PR to try to find the closest valid position.
I think that with this PR, 100% of reports will have a position (at least the closest possible to the problem).

[kkHAIKE]

LGTM