Fix a regression from r211653.

The method was empty in the null streamer but I mistakenly replaced it with
the aborting one in MCStreamer.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211666 91177308-0d34-0410-b5e6-96231b3b80d8
Fix another asserting method in the null streamer.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211668 91177308-0d34-0410-b5e6-96231b3b80d8