Fix heap buffer overflow

Bug: oss-fuzz:11040

Because we're sampling, the offset ends up the same as the width. Back
up to the left enough to fit the bytes we will write.

Change-Id: Ie476a0191b66c2322446b9c0922f630d6e971645
Reviewed-on: https://skia-review.googlesource.com/c/164262
Commit-Queue: Leon Scroggins <scroggo@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
Auto-Submit: Leon Scroggins <scroggo@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
1 file changed