[KUrlNavigatorButton] Fix QString usage to not use [] out of bounds.

Summary:
Since Qt-5.14 this leads to a runtime warning from QCharRef.

The solution resizes "big" before hand, and then resizes again at
the end, to the final size.

Test Plan: running dolphin; new unittest

Reviewers: elvisangelaccio

Reviewed By: elvisangelaccio

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D23898