Avoid chopping cubics with w=0

The new test case (and the linked fuzz) were bumping into the assert
added here: https://review.skia.org/667436

Bug: oss-fuzz:511244869
Change-Id: I6623534671a431b4d8dde3972da9ead36842a5ad
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1253616
Reviewed-by: Florin Malita <fmalita@google.com>
Commit-Queue: Kaylee Lubick <kjlubick@google.com>