[jit] Minor: in unpickler, string tweak in readBytes() (#35550)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/35550
Avoid clearing data before copying into the string buffer in a few cases.
ghstack-source-id: 101020139
Test Plan: buck test mode/dev-nosan caffe2/test/cpp/jit/...
Differential Revision: D20699725
fbshipit-source-id: 14dce40dbebdd64fd0d60372cad1b642602205db
