guard against nullptr while drawing

I was unable to reproduce the fuzz, but
DirectMaskSubRunNoCache::Make can certainly return nullptr.
Draw nothing when a nullptr is passed.

Bug: oss-fuzz:33618

Change-Id: I6aaa954b87c7d2b19fa96ff5d66de4083a589580
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/401516
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
1 file changed