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If you were writing assembly that would be reliable, but in C it isn't. The compiler knows that NULL dereferences are undefined behaviour and so won't necessarily compile them into memory accesses.

Yeah, in C you need to use assertions (or simple checks) for things that might be null.

That said the compiler isn't infinitely smart (thank god) and complex null derefs will "safely" make it to runtime. (What a sad world we're in.)

A good optimizing compiler can remove the dereference entirely if it notices, unfortunately.

How big is this MPU region? Pointers to large structs can easily allow access past the protected region.