Fixing possible out of bound memory access

This was a bug found by ASAN. When width is very small, we can have something like width == 1 and rowBytes == 8. Using "2 * yWidth" (2) would be smaller than rowBytesY (8), so we could read memory out of bounds. This issue has a separate fix in blink (crbug.com/458861).

BUG=skia:

Review URL: https://codereview.chromium.org/936133003
1 file changed