Because you write out-of-bounds values (val < 0 or val > 255) or out-of-bounds pixels (pix < 0 or pix > NUM_PIX - 1)... writing to NUM_PIX is out-of-bounds due to the counting system (0 .. n-1).
Because you write out-of-bounds values (val < 0 or val > 255) or out-of-bounds pixels (pix < 0 or pix > NUM_PIX - 1)... writing to NUM_PIX is out-of-bounds due to the counting system (0 .. n-1).