devmapper: Fix MountDevice for non-existing Device

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)

graphdriver/devmapper/deviceset.go

@@ -804,6 +804,9 @@ func (devices *DeviceSet) MountDevice(hash, path string) error {
 	defer devices.Unlock()
 
 	info := devices.Devices[hash]
+	if info == nil {
+		return fmt.Errorf("Unknown device %s", hash)
+	}
 
 	if info.mountCount > 0 {
 		if path != info.mountPath {