Steps to reproduce the crash:
 1. Open some long file, e.g. botan.cpp
 2. Trigger completion and close editor immediately

The IDocument pointer was only used to reference the file name. Instead
of passing an IDocument* to the IAssistInterface, pass the file name.

Change-Id: Iafce9b818806a77968a10541114bc9b7c8665f11
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>
Reviewed-by: David Schulz <david.schulz@digia.com>