Catch SkRuntimeEffects w/o "main" earlier

Previously, these would produce a "valid" effect object, but it wouldn't
draw anything.

Bug: oss-fuzz:24070
Change-Id: I17d0ed1710196853da0694cac9f4c260312700a9
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/304064
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
2 files changed