crypto/ssh: fix several logic errors.
crypto/ssh: fix several logic errors.

These are the obvious mistakes from my read through. I'll save the
more cosmetic changes for a later CL.

R=dave, kardianos
CC=golang-dev
http://codereview.appspot.com/6049050

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