Fix signed integer overflow in rng_test.h (#36421)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/36421
Test Plan: Imported from OSS
Differential Revision: D20978925
Pulled By: pbelevich
fbshipit-source-id: 30b6abb19abe70738a3a68427f3b3df67510fb48
