Fix div-by-zero loophole in gradient factory func

Bug: oss-fuzz:10373
Change-Id: I4277fb63e3186ee34feaf09ecf6aeddeb532f9c1
Reviewed-on: https://skia-review.googlesource.com/c/168269
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
2 files changed