docker: Close pipe when the docker API call fails

This hasn't been found in the wild but by code inspection. If we fail the
docker API call, the pipe is never closed. Close it before returning.

probe/docker/controls.go

@@ -86,6 +86,7 @@ func (r *registry) attachContainer(containerID string, req xfer.Request) xfer.Re
 		ErrorStream:  local,
 	})
 	if err != nil {
+		pipe.Close()
 		return xfer.ResponseError(err)
 	}
 	pipe.OnClose(func() {
@@ -133,6 +134,7 @@ func (r *registry) execContainer(containerID string, req xfer.Request) xfer.Resp
 		ErrorStream:  local,
 	})
 	if err != nil {
+		pipe.Close()
 		return xfer.ResponseError(err)
 	}