Fix crash introduced with a7ba6eb0

Summary:
Reason for the crash: with "Quick Open: Remove "file:///" prefix from file paths"
(a7ba6eb0) 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