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
https://golang.org/cl/6175047

src/pkg/crypto/x509/x509.go

@@ -388,10 +388,10 @@ func (c *Certificate) CheckSignature(algo SignatureAlgorithm, signed, signature
 		return ErrUnsupportedAlgorithm
 	}
 
-	h := hashType.New()
-	if h == nil {
+	if !hashType.Available() {
 		return ErrUnsupportedAlgorithm
 	}
+	h := hashType.New()
 
 	h.Write(signed)
 	digest := h.Sum(nil)