go.crypto/ssh: fix misplaced defer
go.crypto/ssh: fix misplaced defer

Fixes issue 3972.

R=golang-dev, agl, r
CC=golang-dev
http://codereview.appspot.com/6448166

drhax9908/go.crypto,isislovecruft/go.crypto,isislovecruft/go.crypto,tonnerre/golang-go.crypto,polydawn-ports/go.crypto,cmars/sshlurp-go.crypto,dizzyd/go.crypto,xman1101/terminal,xman1101/terminal,polydawn-ports/go.crypto,cmars/go.crypto,dizzyd/go.crypto,tonnerre/golang-go.crypto,cmars/go.crypto,cmars/sshlurp-go.crypto,drhax9908/go.crypto