free(nullptr) does not violate the nofree specification

This fixes a subtle and nasty bug in my 86664638. The problem is that free(nullptr) is well defined (and common).

The specification for the nofree attributes talks about memory objects, and doesn't explicitly address null, but I think it's reasonable to assume that nofree doesn't disallow a call to free(nullptr). If it did, we'd have to prove nonnull on an argument to ever infer nofree which doesn't seem to be the intent.

This was found by Nuno and Alive2 over in https://reviews.llvm.org/D100141#2697374.

Differential Revision: https://reviews.llvm.org/D100779