crypto/tls: disallow handshake messages fragmented across CCS

Detected by BoGo test FragmentAcrossChangeCipherSpec-Server-Packed.

Change-Id: I9a76697b9cdeb010642766041971de5c7e533481
Reviewed-on: https://go-review.googlesource.com/48811
Reviewed-by: Adam Langley <agl@golang.org>
Run-TryBot: Adam Langley <agl@golang.org>

conn.go

@@ -686,6 +686,11 @@ Again:
 			c.in.setErrorLocked(c.sendAlert(alertUnexpectedMessage))
 			break
 		}
+		// Handshake messages are not allowed to fragment across the CCS
+		if c.hand.Len() > 0 {
+			c.in.setErrorLocked(c.sendAlert(alertUnexpectedMessage))
+			break
+		}
 		err := c.in.changeCipherSpec()
 		if err != nil {
 			c.in.setErrorLocked(c.sendAlert(err.(alert)))