crypto/tls: reject ServerHellos with empty ALPN protocols.

https://tools.ietf.org/html/rfc7301#section-3.1 specifies that a
ProtocolName may not be empty. This change enforces this for ServerHello
messages—it's already enforced for ClientHello messages.

Change-Id: Ic5a5be6bebf07fba90a3cabd10b07ab7b4337f53
Reviewed-on: https://go-review.googlesource.com/12003Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/crypto/tls/handshake_messages.go

@@ -763,6 +763,10 @@ func (m *serverHelloMsg) unmarshal(data []byte) bool {
 				return false
 			}
 			d = d[1:]
+			if len(d) == 0 {
+				// ALPN protocols must not be empty.
+				return false
+			}
 			m.alpnProtocol = string(d)
 		case extensionSCT:
 			d := data[:length]