[4fGradient] Fix degenerate vertical gradient assert

[4fGradient] Fix degenerate vertical gradient assert

For vertical gradients, we rely on LinearIntervalProcessor to yield a
fAdvX == +inf, thanks to division by dx (== +/-0).

But certain degenerate values may cause the numerator to also collapse
to 0, resulting in fAdvX == NaN.

Instead or relying on float semantics, pin fAdvX to +inf explicitly for
vertical gradients.

BUG=skia:5912

Change-Id: Ia8007b99802a6b1e0b22f5618a0ca8959b0cfbb2
Reviewed-on: https://skia-review.googlesource.com/8223
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>