Fix fuzzer-discovered null deref with empty interface blocks.

Change-Id: I95bfa33f674bce0098cc6d4469c8c85ee8b051f5
Fixes: oss-fuzz:38875
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/449817
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
3 files changed