context: don't return a nil Cause for a canceled custom context
Avoid a case where Cause(ctx) could return nil for a canceled context,
when ctx is a custom context implementation and descends from a
cancellable-but-not-canceled first-party Context.
Fixes #73258
Change-Id: Idbd81ccddea82ecabece4373d718baae6ca4b58e
Reviewed-on: https://go-review.googlesource.com/c/go/+/663936 Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>