Fix potential crash when aborting a ListJob

Libarchive's error() signal is connected to the Job::onError slot, but
when we abort a ListJob, the slot might be called when the job instance
has already been deleted. So we now emit this signal only when we are not
aborting the job.

BUG: 369283
FIXED-IN: 16.08.2

Differential Revision: D2852