[Object] Don't crash on empty export lists.

Summary: This fixes the exports iterator if the export list is empty.

Reviewers: Bigcheese, kledzik

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6732

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224563 91177308-0d34-0410-b5e6-96231b3b80d8