do not attempt to kill a process which never started

it's difficult to trigger such an error, but if taskCmd.Start() actually fails, it never sets taskCmd.Process, so trying to use the PID there is a nil access and causes executor to crash.

Author: knopers8

executor/executable/controllabletask.go

@@ -147,8 +147,6 @@ func (t *ControllableTask) doLaunchTask(taskCmd *exec.Cmd, launchStartTime time.
 			Error("failed to run task")
 
 		t.sendStatus(t.knownEnvironmentId, mesos.TASK_FAILED, err.Error())
-		// fixme: i confirmed on staging, that's a nil access! taskCmd.Process is not set if Start() fails
-		_ = t.doTermIntKill(-taskCmd.Process.Pid)
 		// fixme: shouldn't we also close pipes, as we do in some other error cases later?
 		return
 	}