refactor: make s2n_array_len constant

[lrstewart]

Description of changes:

Our grep_simple_mistakes script requires that we use s2n_array_len everywhere instead of (sizeof(array) / sizeof(array[0])), but s2n_array_len is no longer defined as (sizeof(array) / sizeof(array[0])), so it's no longer constant. That means that you can't do like:

struct s2n_blob array1[100] = { 0 };
struct s2n_blob array2[s2n_array_len(array1)] = { 0 };

You could do (sizeof(array) / sizeof(struct s2n_blob)) as a workaround, but that's unnecessary and more brittle.

The null check is also unnecessary because arrays can't be null. Only pointers can be null, and s2n_array_len doesn't work on pointers anyway. You could argue that we need a sizeof(array) > 0 check instead, but funnily enough array[0] actually doesn't seg fault for an empty array: https://godbolt.org/z/s4vcrr6fP. I guess that makes sense, since you don't have to dereference anything. I'd hope at least some compilers or static analyzers would yell at you though.

Testing:

I'm just removing an unnecessary null check from a macro. I'm not sure how I'd test this.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.