Fix crash introduced with a7ba6eb00095df86ae6bc482932

Summary:
Reason for the crash: with "Quick Open: Remove "file:///" prefix from file paths"
(a7ba6eb00095df86ae6bc4829327a41e521f1c1c) the Url prefix file:///
was removed and therefore we hit an assertion (missing protocol) when
trying to open a file from the Quick Open view.

Reviewers: Kate, cullmann

Reviewed By: Kate, cullmann

Subscribers: cullmann, kwrite-devel

Tags: Kate

Differential Revision: https://phabricator.kde.org/D16831