runtime: panic on AddCleanup with self pointer

runtime: panic on AddCleanup with self pointer

For #75066

Change-Id: Ifd555586fb448e7510fed16372648bdd7ec0ab4a
Reviewed-on: https://go-review.googlesource.com/c/go/+/697535
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>