Check p before using it. The issue is when you have code like the following, where the behavior of the first statement is only defined when the condition in the second is false:

    int x = *p;
    if (p == NULL) {
      // p can't be NULL without having triggered
      // undefined behavior in the first line, so
      // this code is removed by the compiler.
      return;
    }
    // ...

The fix is to stop dereferencing p before you know whether it's NULL or not:

    if (p == NULL) {
      // Moving the check before the dereference
      // avoids undefined behavior and resolves
      // the issue.
      return;
    }
    int x = *p;
    // ...