It's valid to address it, but not to see what's there. (The really short answer is, "Yes, so don't worry about it.") With most compilers, if you say

int i, a[MAX], j;

then either i or j is at the part of memory just after the last element of the array. The way to see whether i or j follows the array is to compare their addresses with that of the element following the array. The way to say this in C is that either

& i == & a[ MAX ]

is true or

& a[ MAX ] == & j

is true. This isn't guaranteed; it's just the way it usually works.