[llvm-objcopy] Fix null symbol handling

This fixes the bug where strip-all option was
leading to a malformed outputted ELF file.

Differential Revision: https://reviews.llvm.org/D47414

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@333772 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed