In case of negative y in the region to process, we were accessing invalid memory
(negative array index).
I hesitated between make so that a given ordinate always use the same index or
if we just want the start ordinate (whatever it is) to use index 0. The later
could have just been `(y - result->y) % RANDOM_TABLE_SIZE`.
I just decided to keep the existing logic (former case) though to be fair, not
sure it matters much.
(cherry picked from commit a86560bb57)
7f29543895