C++: Fix FPs in cpp/overflow-buffer

[MathiasVP]

Consider this snippet:

struct S {
  uint8_t a;
  uint8_t b;
  uint8_t c;

  void test() {
    memset(&b, 0, sizeof(S) - offsetof(S, b));
  };
};

The cpp/overflow-buffer query flags this up claiming that we overrun the buffer b (of size 8) because the memset may access 16 bytes starting at the address &b. However, this access is fine, and this is just a fancy way of zero'ing b and c.

This PR fixes this by modifying the initial size deduced from &b so that it correctly reflects that it's fine to access 16 bytes starting at this address.

Commit-by-commit review encouraged.

While DCA appears uneventful, this appears to solve quite a few FPs at Microsoft 🎉

[Copilot]

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cpp/ql/src/change-notes/2025-01-28-overflow-buffer.md:2

• [nitpick] The category name 'minorAnalysis' could be clearer or more standardized. Consider using a recognized category name or a more descriptive identifier.

category: minorAnalysis

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[geoffw0]

Change LGTM.