Verifier: Don't crash on null entries in debug info retained types list

There was already a good error path for this. Added a test for it & made
a minor code change to ensure the error path was actually reached,
rather than crashing before we got that far.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@245795 91177308-0d34-0410-b5e6-96231b3b80d8