Add defensive check that argument may be null in call to compareConversionFunctions() on incorrect code.

I'm looking into getting a reduced test case, but it's not
immediately available.

Fixes <rdar://problem/16344806>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205285 91177308-0d34-0410-b5e6-96231b3b80d8