llvm-objdump: prevent out of bounds accesses during unwind dumping.

We were a bit too trusting about the offsets encoded in MachO compact unwind
sections, so this passes every access through a bounds check just in case. It
prevents a few segfaults on malformed object files, if one should ever come
along.

Mostly to silence fuzzers in the vague hope they might be able to produce
something useful without the noise.

llvm-svn: 323198