nodedev: Add check for NULL obj before call Unlock

Commit id '95ea171b' was a bit too aggressive in removing the if (obj)
check since cleanup is reachable after Unlock and obj = NULL.

src/node_device/node_device_driver.c

@@ -650,7 +650,8 @@ nodeDeviceDestroy(virNodeDevicePtr dev)
 
  cleanup:
     nodeDeviceUnlock();
-    virNodeDeviceObjUnlock(obj);
+    if (obj)
+        virNodeDeviceObjUnlock(obj);
     VIR_FREE(wwnn);
     VIR_FREE(wwpn);
     return ret;