fix(consumer): guard against nil client

Quite minor, but passing a nil client into NewConsumerGroupFromClient
would cause a panic. Add a simple guard to prevent that.

Signed-off-by: Dominic Evans <dominic.evans@uk.ibm.com>

consumer_group.go

@@ -114,6 +114,9 @@ func NewConsumerGroup(addrs []string, groupID string, config *Config) (ConsumerG
 // necessary to call Close() on the underlying client when shutting down this consumer.
 // PLEASE NOTE: consumer groups can only re-use but not share clients.
 func NewConsumerGroupFromClient(groupID string, client Client) (ConsumerGroup, error) {
+	if client == nil {
+		return nil, ConfigurationError("client must not be nil")
+	}
 	// For clients passed in by the client, ensure we don't
 	// call Close() on it.
 	cli := &nopCloserClient{client}

consumer_group_test.go

@@ -27,6 +27,14 @@ func (h *handler) ConsumeClaim(sess ConsumerGroupSession, claim ConsumerGroupCla
 	return nil
 }
 
+func TestNewConsumerGroupFromClient(t *testing.T) {
+	t.Run("should not permit nil client", func(t *testing.T) {
+		group, err := NewConsumerGroupFromClient("group", nil)
+		assert.Nil(t, group)
+		assert.Error(t, err)
+	})
+}
+
 // TestConsumerGroupNewSessionDuringOffsetLoad ensures that the consumer group
 // will retry Join and Sync group operations, if it receives a temporary
 // OffsetsLoadInProgress error response, in the same way as it would for a