[llvm-readobj] Remove dead code. Add an assertion instead.

[llvm-readobj] Remove dead code. Add an assertion instead.

When we arrive at the end of the function, the validation of
the object has been done already. In theory, so, we should never
arrive here with something broken as the object isn't mutated.
Practice sometimes proves theory to be wrong, so leave an assertion
instead, as suggested by David Blaikie, to catch bugs.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@257570 91177308-0d34-0410-b5e6-96231b3b80d8