Fix C++ defensive coding advice from the QUICv2 CL (cl/424961568).
Fix C++ defensive coding advice from the QUICv2 CL (cl/424961568).

There is much more to be done, but that could get out of control fast. Let me know if you want to abandon this, push more into this CL, or commit as-is.

PiperOrigin-RevId: 425625946

google/quiche,google/quiche,google/quiche,google/quiche