Rice's theorem is a bit more direct here: C is Turing complete, some C programs derefernce null pointers, and some don't. Therefore, the question of whether a an arbitrary C program derefernces a null pointer is in general undecidable.

But I don't think the Op was claiming it was possible to do this perfectly, just possible to write very useful tools. You will always have either flase positives or false negatives (or I suppose inputs where you just hang).

Turing completeness is the bane of static analysis, but that doesn't make it a fruitless endeavor.