NULL-based reference.
Note: Encountered this a few times on Tiger + gcc 4.0.1. Might just be a
platform-specific compiler issue, but it's good defensive programming in any
case.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@59890 91177308-0d34-0410-b5e6-96231b3b80d8