Fix overflow bug in slow span.

Fix an overflow in the address calculation in the sampler.

BUG=chromium:700836

TBR=hcm@google.com

Change-Id: Ifadbdc9541138219e8aec08c1342a241da75705c
Reviewed-on: https://skia-review.googlesource.com/9815
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-on: https://skia-review.googlesource.com/10479
Reviewed-by: Herb Derby <herb@google.com>
1 file changed