zero bits for trailer

[tlyu]

Use zero bits, not ones, for the trailer, to avoid lighting up downstream pixels when the DotStar object is created with fewer pixels than are physically present in the chian.

This is visible after the second update of a DotStar object that is at least 17 pixels long, when there is a greater number of physical pixels in the chain.

[tlyu]

See https://forums.adafruit.com/viewtopic.php?t=192301 for an example of confusing behavior experienced by a user due to this bug. I outlined the fix in the thread, but only recently acquired a DotStar strip to test with.

[FoamyGuy]

This change looks good to me. I don't have any prior experience or knowledge of the details of the protocol that drives dotstars, but I did read through the forum thread and the blogpost mentioned in it and did see the section referencing sending the trailer zeros instead of 1's. The change here does appear to me to follow the behavior put forth by that resource.

I tested the change successfully using the reproducer code from the forums:

import board, adafruit_dotstar as dotstar, time
n = 17
dots = dotstar.DotStar(board.SCK, board.MOSI, n, brightness=0.2)
for i in range(dots.n):
    dots[i] = (0x66, 0x33, 0x99, )  # Rebecca Purple
time.sleep(1)
dots.deinit()
print('done...')

I did reproduce the issue mentioned, and verified that the changed version resolves it, leaving no extra LEDs on after it deinits.

Thank you @tlyu for the fix!