Add assertions and checks to album art fetch code

Summary:
Add a couple of assertions and checks to the album art fetch code. Hopefully fixes bug https://bugs.kde.org/show_bug.cgi?id=393274, or otherwise at least gives a bit more information.

BUG 393274

Test Plan: Fixes the bug, according to the reporter.

Reviewers: KDE Connect, albertvaka

Reviewed By: KDE Connect, albertvaka

Subscribers: albertvaka, KDE Connect

Tags: KDE Connect

Differential Revision: https://phabricator.kde.org/D12618