Don't return null-ish method names (#1508)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1508

Ensure that the Program API can't return an invalid pointer for a method name when the program file is corrupt. Without this check, it could return a pointer like `0x0004`, causing a segfault when trying to compare against it or print it.

Reviewed By: JacobSzwejbka

Differential Revision: D52451744

fbshipit-source-id: 9158d693f14a723a34f9fcbd1f7395224b7b76b5