crypto/x509: fix panic when using unavailable hash function.
crypto/x509: fix panic when using unavailable hash function.

crypto.Hash.New() changed to panicking when the hash function isn't
linked in, but crypto/x509 still expects it to return nil.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/6175047

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