[JENKINS-73726] Avoid NPE trying to cancel a block which is already done

[jglick]

Amending #925. Test case is in jenkinsci/pipeline-model-definition-plugin#738; did not manage to reproduce without Declarative (something weird in ModelInterpreter.getParallelStages).

workflow-cps-plugin/plugin/src/main/java/org/jenkinsci/plugins/workflow/cps/steps/ParallelStep.java

Line 151 in 3761c26

handler.stepExecution.stop(new FlowInterruptedException(Result.ABORTED, true, new FailFastCause(name)));

is immediately responsible:

org.jenkinsci.plugins.workflow.steps.FlowInterruptedException
	at org.jenkinsci.plugins.workflow.cps.steps.ParallelStep$ResultHandler$Callback.checkAllDone(ParallelStep.java:151)
	at org.jenkinsci.plugins.workflow.cps.steps.ParallelStep$ResultHandler$Callback.onFailure(ParallelStep.java:138)
	at org.jenkinsci.plugins.workflow.cps.CpsBodyExecution$FailureAdapter.receive(CpsBodyExecution.java:357)
	at com.cloudbees.groovy.cps.impl.ThrowBlock$1.receive(ThrowBlock.java:66)
	at com.cloudbees.groovy.cps.impl.ConstantBlock.eval(ConstantBlock.java:21)
	at com.cloudbees.groovy.cps.Next.step(Next.java:83)
	at com.cloudbees.groovy.cps.Continuable.run0(Continuable.java:147)
	at …

java.lang.Exception: Stack trace
	at java.base/java.lang.Thread.dumpStack(Thread.java:2209)
	at org.jenkinsci.plugins.workflow.cps.CpsBodyExecution.cancel(CpsBodyExecution.java:251)
	at org.jenkinsci.plugins.workflow.cps.steps.ParallelStepExecution.stop(ParallelStepExecution.java:67)
	at org.jenkinsci.plugins.workflow.cps.steps.ParallelStep$ResultHandler$Callback.checkAllDone(ParallelStep.java:151)
	at org.jenkinsci.plugins.workflow.cps.steps.ParallelStep$ResultHandler$Callback.onFailure(ParallelStep.java:138)
	at org.jenkinsci.plugins.workflow.cps.CpsBodyExecution$FailureAdapter.receive(CpsBodyExecution.java:357)
	at com.cloudbees.groovy.cps.impl.ThrowBlock$1.receive(ThrowBlock.java:66)
	at com.cloudbees.groovy.cps.impl.ConstantBlock.eval(ConstantBlock.java:21)
	at com.cloudbees.groovy.cps.Next.step(Next.java:83)
	at com.cloudbees.groovy.cps.Continuable.run0(Continuable.java:147)
	at …