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

pombredanne/go-deleteme,ssrl/go,ssrl/go,dsriram/golang,h8liu/golang,apparentlymart/golang,h8liu/golang,apparentlymart/golang,tv42/old-go,dsriram/golang,pombredanne/go-deleteme,apparentlymart/golang,ssrl/go,h8liu/golang,apparentlymart/golang,pombredanne/go-deleteme,gnanderson/go,dsriram/golang,tv42/old-go,gnanderson/go,apparentlymart/golang,pombredanne/go-deleteme,h8liu/golang,h8liu/golang,gnanderson/go,apparentlymart/golang,gnanderson/go,ssrl/go,apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,dsriram/golang,pombredanne/go-deleteme,tv42/old-go,h8liu/golang,ssrl/go,tv42/old-go,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,ssrl/go,dsriram/golang,gnanderson/go,dsriram/golang,tv42/old-go,h8liu/golang,ssrl/go,gnanderson/go,dsriram/golang