Don't crash after Cancel in Job Already Running dialog

When a user Execute-Launches an application a second time while the
previously launched instance of this application is still running, the
"Job Already Running" dialog appears with 3 buttons. If a user clicks
the Cancel button, a NativeAppJob kills itself Quietly. This
NativeAppJob belongs to an ExecuteCompositeJob, which is not notified
when a subjob is killed Quietly. So the ExecuteCompositeJob keeps
waiting for it to finish. When this waiting ExecuteCompositeJob is
killed (e.g. via "Stop All" button or on KDevelop exit), it attempts to
kill the subjob it believes is still running, but which in fact is long
since destroyed. This usually causes a segmentation fault.

Aleix Pol fixed a similar crash when the "Kill All Instances" button in
the "Job Already Running" dialog is clicked in

BUG: 399511, 416874
FIXED-IN: 5.6.1
1 job for !167 with job-already-running-cancel-crash-fix in 60 minutes and 25 seconds (queued for 22 minutes and 51 seconds)
Status Job ID Name Coverage
failed #69187
linux kf5-qt5 qt5.12


Name Stage Failure
linux kf5-qt5 qt5.12 Build The script exceeded the maximum execution time set for the job
                                              14      10    71%   61,63-65
plugins/welcomepage/welcomepageview.cpp 26 25 96% 65
TOTAL 141702 64288 45%
Uploading artifacts for successful job
Uploading artifacts...
build: found 16330 matching files and directories

ERROR: Job failed: execution took longer than 1h0m0s seconds