Be sure that every absolute path is relative to portal root

I found another issue with `findObject` function.

If the relative path starts with `/siteid/foo/bar` it's translated to `foo/bar`.

But if it starts with `/foo/bar` (what I get with Plone behind apache) it was returning `/foo/bar`.
This leading slash character make the first `child_id` element popped from the `components` list to be an empty string, and this let the `while` exits immediatly.

The result: reference is not kept. Possible that no-one have found this before? It seems has been added there: https://github.com/plone/plone.app.linkintegrity/commit/c281e9892b931bde53ed53f3393ad4c6c25ad537.

As for PR 12 or 16: this must be probably fixed for master also. Let me know.