According to the manual both store_result and use_result should only return false and an object. But for me the manual have been wrong so many times so I would suggest that you verify that is the case. For example in this case we have to return null in some cases which isn't documented. Just testing doing a normal UPDATE (and not multi query stuff) and then call store_result/use_result to see that you get FALSE back and not true.

I'm not on the HHVM team anymore so I don't have time to do this unfortunately. Joel may be able to but I bet he would be very happy if you tested it and submitted a new diff that changed return $result; to return false in both store_result and use_result.