Nothing stops you from having your null pointer in the middle of the address space. Some C compiler for DOS or early Windows did that IIRC (it was 0xB800 or something?.. so that it wouldn't accidentally corrupt the interrupt table). Also, C explicitly prohibits address wrap-around problems for pointers:

    Moreover, if the expression P points to the last element of an array object, the expression (P)+1 points one past the last element of the array object, and if the expression Q points one past the last element of an array object, the expression (Q)-1 points to the last element of the array object. If both the pointer operand and the result point to elements of the same array object, or one past the last element of the array object, the evaluation shall not produce an overflow; otherwise, the behavior is undefined.

But this is fine, since pointer comparisons (as in, less/greater comparisons) are actually both pretty restricted and required to have reasonable semantics when comparing pointers that point into the same object/array:

    When two pointers are compared, the result depends on the relative locations in the address space of the objects pointed to. If two pointers to object types both point to the same object, or both point one past the last element of the same array object, they compare equal. If the objects pointed to are members of the same aggregate object, pointers to structure members declared later compare greater than pointers to members declared earlier in the structure, and pointers to array elements with larger subscript values compare greater than pointers to elements of the same array with lower subscript values. All pointers to members of the same union object compare equal. If the expression P points to an element of an array object and the expression Q points to the last element of the same array object, the pointer expression Q+1 compares greater than P. In all other cases, the behavior is undefined.

By the way, this means that, among other things, if you use number N to represent a null pointer then number N-1 can not ever be a valid pointer to anything: adding 1 to a valid pointer is always allowed, and this addition should produce a non-null pointer — because the resulting pointer is required to be well-behaved in comparisons, and comparisons with null pointer are UB.