runtime: fix unwindstack crash
runtime: fix unwindstack crash

Bug and fix identified by Alexey Gokhberg.
Fixes issue 1135.

R=r, brainman
CC=golang-dev
http://codereview.appspot.com/2198046
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go