llvm-objdump: Handle BSS sections larger than the object file

The size of the uninitialized sections, like BSS, can exceed the size of
the object file.

Do not attempt to grab the contents of such sections.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212953 91177308-0d34-0410-b5e6-96231b3b80d8