Fix BB after r354328.

Bot:
http://lab.llvm.org:8011/builders/lld-x86_64-darwin13/builds/30188/steps/build_Lld/logs/stdio

Error:
/Users/buildslave/as-bldslv9_new/lld-x86_64-darwin13/llvm.src/lib/ObjectYAML/ELFYAML.cpp:1013:15: error: unused variable 'Object' [-Werror,-Wunused-variable]
  const auto *Object = static_cast<ELFYAML::Object *>(IO.getContext());
              ^
/Users/buildslave/as-bldslv9_new/lld-x86_64-darwin13/llvm.src/lib/ObjectYAML/ELFYAML.cpp:1023:15: error: unused variable 'Object' [-Werror,-Wunused-variable]
  const auto *Object = static_cast<ELFYAML::Object *>(IO.getContext());

Fix:
change 
  const auto *Object = static_cast<ELFYAML::Object *>(IO.getContext());
  assert(Object && "The IO context is not initialized");
to
  assert(!IO.getContext() && "The IO context is initialized already");

llvm-svn: 354329