[llvm-pdbdump] Add guard for null pointers and remove unused code

Summary: This avoids crashing when a user tries to dump a pdb with the `-native` option.

Reviewers: zturner, llvm-commits, rnk

Reviewed By: zturner

Subscribers: mgrang

Differential Revision: https://reviews.llvm.org/D44117

llvm-svn: 326863