llvm-objdump: deal with unexpected object files more gracefully.

Specifically, we don't want to segfault on release builds, so print the problem
instead.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@287022 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed