kzrdude is referring to the allocation of the Array struct on the heap. It should be something like this instead:

    Array array_create(size_t size, size_t sizeof_data) {
        Array result;
        result.size = size;
        result.capacity = size;
        if(size) {
            result.data = malloc(size * sizeof_data);
        } else {
            result.data = NULL;
        }
        return result;
    }