Fixed connection dispose bug before connection connected.

src/DotNetCore.CAP.Kafka/KafkaConsumerClient.cs

@@ -75,7 +75,7 @@ namespace DotNetCore.CAP.Kafka
 
         public void Dispose()
         {
-            _consumerClient.Dispose();
+            _consumerClient?.Dispose();
         }
 
         #region private methods

src/DotNetCore.CAP.RabbitMQ/RabbitMQConsumerClient.cs

@@ -89,8 +89,8 @@ namespace DotNetCore.CAP.RabbitMQ
 
         public void Dispose()
         {
-            _channel.Dispose();
-            _connection.Dispose();
+            _channel?.Dispose();
+            _connection?.Dispose();
         }
 
         #region events