Fix bug which may be causing the fits crashes.

Summary:
It looks like the thread which is writing the fits files is using memory (QString filename) that is a
local variable in a method which likely ends before the write thread ends.
Change this filename variable to a class variable which should be safe memory.

Reviewers: mutlaqja

Reviewed By: mutlaqja

Subscribers: kde-edu

Tags: KDE Edu

Differential Revision: https://phabricator.kde.org/D27115