[caffe2] fix LLVM-12 nullptr-with-nonzero-offset UBSAN error (#65506)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/65506
Test Plan: run a adfinder canary and verify this error is fixed.
Reviewed By: swolchok
Differential Revision: D31130083
fbshipit-source-id: c31f179f8a7de75ed6f6e7ee68b197f2970ddd3d
