Fix race condition in processJobs()

Fix race condition in processJobs()

Don't modify list of running jobs when a job is aborted since the job
may have just started. Wait till the next time processJobs() is invoked
for a new job and discard the aborted job at that time.

(cherry picked from commit f55ecc080d0c5eca4e65a235c63ab13867c86874
in 4.8)

Task-number: QTBUG-20841
Change-Id: Icb4cd089505c0634d3fec023b52c61bbc878404f
Reviewed-on: http://codereview.qt.nokia.com/2837
Reviewed-by: Bea Lam <bea.lam@nokia.com>
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>