Don't calculate attribute() twice.

Summary:
"kateTextLine" neither "i" shouldn't have changed since the first call to attribute two lines ago.

Test Plan: Open a XML file with 4 lines and a line of 566039 characters long.

Reviewers: Frameworks, Kate, dhaumann

Reviewed By: Kate, dhaumann

Subscribers: dhaumann

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D11491