crypto/tls: fix ServerHello SCT test

According to https://tools.ietf.org/html/rfc6962#section-3.3, the SCT
must be at least one byte long. The parsing code correctly checks for
this condition, but rarely the test does generate an empty SCT.

Change-Id: If36a34985b4470a5a9f96affc159195c04f6bfad
Reviewed-on: https://go-review.googlesource.com/c/129755Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/crypto/tls/handshake_messages_test.go

@@ -188,7 +188,7 @@ func (*serverHelloMsg) Generate(rand *rand.Rand, size int) reflect.Value {
 		numSCTs := rand.Intn(4)
 		m.scts = make([][]byte, numSCTs)
 		for i := range m.scts {
-			m.scts[i] = randomBytes(rand.Intn(500), rand)
+			m.scts[i] = randomBytes(rand.Intn(500)+1, rand)
 		}
 	}