Add an assert to further check the invariant that a null pointer

Add an assert to further check the invariant that a null pointer
corresponds to another argument being valid.

This makes it clear that the code is correct despite the PVS-Studio
report that a pointer might be dereferenced prior to being checked for
whether it is null. It likely is also enough for static analyzers to not
flag the code.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@285982 91177308-0d34-0410-b5e6-96231b3b80d8