C's notion of null _is_ the _only_ relevant

Some undefined behaviour does require a diagnostic, but the universe of
undefined behaviour is unbounded, and determining if some things are
undefined rams you right into Rice's theorem and the halting problem.

Spotting null dereferences in the general case certainly is (although
warning when the compiler *already* spots a null dereference, as you
propose, is not hard: have you thought about the case of NULL dereferences
being brought into a function via inlining, though? I'd try a GCC using
this warning on a large template-heavy C++ codebase before considering it:
see how many FPs you get.)