[release-branch.go1.3] runtime: add test for issue 8047.
[release-branch.go1.3] runtime: add test for issue 8047.

««« CL 108840043 / 3a2306461574
runtime: add test for issue 8047.

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

Fixes #8047

LGTM=dvyukov, rsc
R=dvyukov, rsc
CC=golang-codereviews
https://golang.org/cl/108840043

»»»

TBR=adg
CC=golang-codereviews
https://golang.org/cl/102320043

klueska/go-akaros,klueska/go-akaros,klueska/go-akaros,klueska/go-akaros,klueska/go-akaros,klueska/go-akaros,klueska/go-akaros,klueska/go-akaros