Add unit test for error discovered by fuzzer.

Before http://review.skia.org/330743 was submitted, this caused an
assertion during CFG generation: http://screen/95ZaTYzon4bMVtE

Change-Id: Icf93472394de3d17425ad1258a68b263cab88eb1
Bug: oss-fuzz:26759
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/330816
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
3 files changed