crypto/twofish: fix Reset index overflow bug.
crypto/twofish: fix Reset index overflow bug.

Fixes issue 1919.

R=golang-dev, r
CC=golang-dev
http://codereview.appspot.com/4530110

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang