caffe2 | Fix building with `-Wrange-loop-analysis` on
Summary: `-Wrange-loop-analysis` is turned on by default for clang 10 (see https://reviews.llvm.org/D73834). This fixes a warning that's found with that.
Test Plan: Build with clang 10 and check there are no `range-loop-analysis` warnings.
Reviewed By: yinghai
Differential Revision: D22207072
fbshipit-source-id: 858ba8a36c653071eab961cb891ce945faf0fa87
