Check the boundary while cumulating deltas in a mask

Without the fix,

./out/Debug/dm --config t8888 -m complexclip_bw

will trigger the assert.

Note that this boundary check is already in place when deltas are
cumulated without using a mask.

Bug: skia:
Change-Id: I68bfe2b7196d440743d3a7535d097bb8ec7689c3
Reviewed-on: https://skia-review.googlesource.com/113210
Commit-Queue: Yuqian Li <liyuqian@google.com>
Reviewed-by: Cary Clark <caryclark@skia.org>
1 file changed