Fix ubsan in concat_split_op.h (#66283)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/66283
Fixes
```
UndefinedBehaviorSanitizer: nullptr-with-nonzero-offset caffe2/caffe2/operators/concat_split_op.h:185:52
```
Test Plan: Sandcastle
Reviewed By: swolchok
Differential Revision: D31486274
fbshipit-source-id: 20128056f19cf814fdc3e6e144cf9208a4080d6a
