test/stress: fix a goroutine leak in threadRing stresstest
test/stress: fix a goroutine leak in threadRing stresstest

Fixes issue 5527

R=golang-dev, dvyukov
CC=golang-dev
https://codereview.appspot.com/9955043

Committer: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,gnanderson/go,gnanderson/go,pombredanne/go-deleteme,gnanderson/go,gnanderson/go,gnanderson/go,apparentlymart/golang,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,gnanderson/go,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,gnanderson/go,pombredanne/go-deleteme,tv42/old-go