gradient-walker: For NONE repeats, when x < 0 or x > 1, set both colors to 0

gradient-walker: For NONE repeats, when x < 0 or x > 1, set both colors to 0

ec7c9c2b6865b48b8bd14e4 introduced a bug where NONE gradients would be
misrendered, causing the area outside the gradient to be treated as a
(very) long fade to transparent.The problem was that a check for
positions outside the gradients were dropped in favor of relying on
the sentinels.

Aside from misrendering, this also caused a signed integer overflow
when the code would compute a stepper size based on MIN_INT32.

This patches fixes the issue by reinstating a check for these cases
and setting both the right and left colors to transparent black.