If you have a function that references a pointer without checking, then it's valid for the compiler to assume that it is not possible for that pointer to be null in subsequent lines of code, that's the whole point - if it sees that the pointer is used without checks, then this implies that all your other code somehow ensures that at that point it won't ever be null, that you've made sure that null pointers are checked somewhere else.

The compiler often can't verify it (halting problem and friends), but it allows for nice optimizations by assuming that the code as written is actually correct, and the check was skipped intentionally.