Fix possible completed job state issue

[lok52]

Fix possible concurrency issue (reported by @EvgenKor).

Because job.getState method is asynchronous following scenario is possible:

1. We get the job from queue by id
2. We asynchronously get job's state
3. The state is completed
4. Job object that we got in the first step does not contain correct returnvalue because it was not in completed state at the moment of fetching.

Solution:
If we got completed job's state, we need to check job's returnvalue and if it is null re-fetch the job.