Restrict pointer equality to null or valid pointers

In C, pointers can only be tested for equality if they are either null or pointing to a valid object.

The issue names no file, test, or entry point. Start by locating where C pointer equality is accepted or validated, then trace the existing checks for null and valid objects. Done means invalid pointer comparisons are rejected while null and valid-pointer equality remains allowed, with tests covering those cases.

Written by the indexing model from the issue text.