[llvm-objdump] Don't crash when using `-a` on non-archives

This fixes PR39402. The crash was caused when dereferencing nullptr in
DumpObject and printArchiveChild.

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D53690

Patch by Xing GUO


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@345503 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed