Add a workaround for loading broken files with negative frame ids

Summary: CCBUG:395378

Test Plan:
Try to load the file attached to this bugreport:
https://bugs.kde.org/show_bug.cgi?id=395378

Every layer in the file should have two frames (in random order). At least one
of the frames should have sane pixel data

Reviewers: #krita, rempt, jounip

Tags: #krita

Differential Revision: https://phabricator.kde.org/D13562