Fall back on stencil clip when SW mask fails to allocate

When generating a SW clip mask, we didn't used to verify that the
pixels had been successfully allocated. This would obviously crash
when the allocation was not successful. This change checks the
allocation first, and falls back on a stencil clip when it doesn't
succeed.

BUG=656589
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2456153003

Review-Url: https://codereview.chromium.org/2456153003
1 file changed