It seems possible that the .Destroy() function is called on a nil
pointer. This would cause a panic in the node-plugin.
Depending on how far GenVolFromVolID() comes, the returned rbdVolume can
be connected. If an error occurs, the rbdVolume should not be connected
at all, so call the .Destroy() function in those cases too.
Fixes: #4562
Signed-off-by: Niels de Vos <ndevos@ibm.com>