Fix a crash when running 'amarok -l /path/to/file'

Summary: BUG: 410870

Test Plan:
amarok -l <somefile>

Before: Assert "you need to call AmarokConfig::instance before using"

After: the file is queued and played.

Reviewers: heikobecker

Reviewed By: heikobecker

Subscribers: #amarok, amarok-devel

Differential Revision: https://phabricator.kde.org/D23326