Fixed3232 overflow in LinearGradientContext::shadeSpan()

Speculative fix for TAP ubsan failures.

Change-Id: Id3a78adbfab04ba3404a23059cbd8c162d812c2e
Reviewed-on: https://skia-review.googlesource.com/8315
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
3 files changed