client: verify payload of SubscribeMultiple is not empty

MQTT protocol 3.1.1 specifies that the payload in a SUBSCRIBE packet
must not be empty and must contain at least one topic filter and QoS
pair. Validate the input to SubscribeMultiple meets this requirement to
avoid protocol errors when communicating with a broker.

Fixes #384

Signed-off-by: Robert Weber <robertweber95@gmail.com>

client.go

@@ -754,6 +754,10 @@ func (c *client) SubscribeMultiple(filters map[string]byte, callback MessageHand
 			return token
 		}
 	}
+	if len(filters) == 0 {
+		token.setError(fmt.Errorf("topic filter/qos map must not be empty"))
+		return token
+	}
 	sub := packets.NewControlPacket(packets.Subscribe).(*packets.SubscribePacket)
 	if sub.Topics, sub.Qoss, err = validateSubscribeMap(filters); err != nil {
 		token.setError(err)