Fix divide by zero in cubic solver when B was nan

Change-Id: Iac938e40d2c1378d7e86fab502ec26f43a447497
Bug: oss-fuzz:55829
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/641137
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Kevin Lubick <kjlubick@google.com>
2 files changed