Fix a bug in the debayer offset code.

Summary:
When X-offset was given in the fits header, the code caused a read past the end of the buffer.
See https://indilib.org/forum/development/6565-bug-in-fitsdata-cpp/50506.html

Test Plan: fit bayer file with XBAYROFF=1 in the header should no longer read past the end of the buffer.

Reviewers: mutlaqja

Reviewed By: mutlaqja

Subscribers: kde-edu

Tags: KDE Edu

Differential Revision: https://phabricator.kde.org/D27907