This fixes an assertion failure uncovered by the fuzzer.
Bug: oss-fuzz:37469
Change-Id: I626c003cfa8a0bc65851899df3a7695dbe29200b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/441311
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>