Harden callGetPreferredAllocationIfAvailable() return value

Previously, we didn't check the contents of the result after calling out
to the plugin endpoint. This could have resulted in errors if the plugin
returned either 'nil' or an empty result. This patch fixes this.

Author: klueska

pkg/kubelet/cm/devicemanager/manager.go

@@ -997,8 +997,10 @@ func (m *ManagerImpl) callGetPreferredAllocationIfAvailable(podUID, contName, re
 	if err != nil {
 		return nil, fmt.Errorf("device plugin GetPreferredAllocation rpc failed with err: %v", err)
 	}
-	// TODO: Add metrics support for init RPC
-	return sets.NewString(resp.ContainerResponses[0].DeviceIDs...), nil
+	if resp != nil && len(resp.ContainerResponses) > 0 {
+		return sets.NewString(resp.ContainerResponses[0].DeviceIDs...), nil
+	}
+	return sets.NewString(), nil
 }
 
 // sanitizeNodeAllocatable scans through allocatedDevices in the device manager