Trying to avoid an NPE reported by a 1.537 user with the Multijob plugin (patched 1.10).

java.lang.NullPointerException
	at hudson.model.queue.FutureImpl.cancel(FutureImpl.java:78)
	at com.tikal.jenkins.plugins.multijob.MultiJobBuilder.perform(MultiJobBuilder.java:156)
	at hudson.tasks.BuildStepMonitor$1.perform(BuildStepMonitor.java:20)
	at hudson.model.AbstractBuild$AbstractBuildExecution.perform(AbstractBuild.java:781)
	at hudson.model.Build$BuildExecution.build(Build.java:199)
	at hudson.model.Build$BuildExecution.doRun(Build.java:160)
	at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:562)
	at com.tikal.jenkins.plugins.multijob.MultiJobBuild$MultiJobRunnerImpl.run(MultiJobBuild.java:70)
	at hudson.model.Run.execute(Run.java:1665)
	at hudson.model.Run.run(Run.java:1612)
	at com.tikal.jenkins.plugins.multijob.MultiJobBuild.run(MultiJobBuild.java:63)
	at hudson.model.ResourceController.execute(ResourceController.java:88)
	at hudson.model.Executor.run(Executor.java:230)