runtime: add test for issue 8047.
runtime: add test for issue 8047.

Make sure stack copier doesn't barf on a nil defer.
Bug was fixed in https://codereview.appspot.com/101800043
This change just adds a test.

Fixes issue 8047

LGTM=dvyukov, rsc
R=dvyukov, rsc
CC=golang-codereviews
https://codereview.appspot.com/108840043

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang