Amarok crash closing USB device

I have looked into https://bugs.kde.org/show_bug.cgi?id=232051 a bit and there are different places where the crash
happens although one is far more common. I attach a patch with a preliminary
fix. From experience it now crashes 20% of the time when unplugging once
scanning. There might be a drawback to the patch as Amarok crashed once when
unplugging the device long after it had been scanned.

Courtesy: Matthieu Pupat