Fix fuzzer bug in SkDescriptor::isValid

This is following up chrome bug:
https://bugs.chromium.org/p/chromium/issues/detail?id=996211

BUG=skia:18491

Change-Id: I4c85bc916903aaa2fdf7eb226bdf3e57f8b1c66c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/250578
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
2 files changed