MM-54580: Fix panic in jobs/base_workers.go (#24631)

If GetJob fails, we would return a nil job and then try to log
with that. To prevent this, we keep a reference to the old job
and log with that in the error case.

https://mattermost.atlassian.net/browse/MM-54580

```release-note
Fix a panic where a simple worker would crash if if failed to get a job.
```

server/channels/jobs/base_workers.go

@@ -82,13 +82,14 @@ func (worker *SimpleWorker) DoJob(job *model.Job) {
 
 	c := request.EmptyContext(worker.logger)
 
-	var appErr *model.AppError
 	// We get the job again because ClaimJob changes the job status.
-	job, appErr = worker.jobServer.GetJob(c, job.Id)
+	newJob, appErr := worker.jobServer.GetJob(c, job.Id)
 	if appErr != nil {
 		job.Logger.Error("SimpleWorker: job execution error", mlog.Err(appErr))
 		worker.setJobError(job, appErr)
+		return
 	}
+	job = newJob
 
 	err := worker.execute(job)
 	if err != nil {