-strlen should not be called with NULL. Also guarantee that StringRef's Length is 0 if Data is NULL.
-memcmp should not be called with NULL (even if size is 0)
Patch by Matthieu Monrocq!
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@131747 91177308-0d34-0410-b5e6-96231b3b80d8