Fix upgrade path for created resources

This *might* be a fix for #2295 -- I've been unable to reproduce the
bug. However, this guard seems wise regardless. I should never be
overwriting an intialized created resources with a nil.

client/task_runner.go

@@ -247,9 +247,13 @@ func (r *TaskRunner) RestoreState() error {
 	}
 	r.artifactsDownloaded = snap.ArtifactDownloaded
 	r.taskDirBuilt = snap.TaskDirBuilt
-	r.createdResources = snap.CreatedResources
 	r.payloadRendered = snap.PayloadRendered
 
+	// Pre-0.5.3 state snapshots won't have created resources
+	if snap.CreatedResources != nil {
+		r.createdResources = snap.CreatedResources
+	}
+
 	if err := r.setTaskEnv(); err != nil {
 		return fmt.Errorf("client: failed to create task environment for task %q in allocation %q: %v",
 			r.task.Name, r.alloc.ID, err)