Fixed Regression: NPE when checking if the executor is interrupted

29426012ce introduced an NPE.
If a project implements Queue.FlyweightTask, build.getExecutor() returns null.
This prevents any flyweight build from finishing successfully.
Merge pull request #724 from JoelJ/patch-1

[FIXED JENKINS-17025] NPE when checking if the executor is interrupted
[JENKINS-17025] Noting.