GODRIVER-910 don't panic when calling watch on a disconnected client

Change-Id: Ie665018d55c4bdcfa84f472a39d25a5e0a43a4a8

Author: Isabella Siu

mongo/client.go

@@ -531,6 +531,9 @@ func (c *Client) UseSessionWithOptions(ctx context.Context, opts *options.Sessio
 // The client must have read concern majority or no read concern for a change stream to be created successfully.
 func (c *Client) Watch(ctx context.Context, pipeline interface{},
 	opts ...*options.ChangeStreamOptions) (*ChangeStream, error) {
+	if c.topology.SessionPool == nil {
+		return nil, ErrClientDisconnected
+	}
 
 	return newClientChangeStream(ctx, c, pipeline, opts...)
 }

mongo/client_internal_test.go

@@ -536,3 +536,12 @@ func TestClient_Disconnect_NilContext(t *testing.T) {
 	err = c.Disconnect(nil)
 	require.NoError(t, err)
 }
+
+func TestClient_Watch_Disconnected(t *testing.T) {
+	cs := testutil.ConnString(t)
+	c, err := NewClient(options.Client().ApplyURI(cs.String()))
+	require.NoError(t, err)
+	change, err := c.Watch(context.Background(), []bson.D{})
+	require.Nil(t, change)
+	require.Equal(t, err, ErrClientDisconnected)
+}