fix crash when there are attachments with null network

Signed-off-by: Andrey Kolomentsev <andrey.kolomentsev@gmail.com>

manager/allocator/network.go

@@ -1052,6 +1052,9 @@ func (a *Allocator) allocateNode(ctx context.Context, node *api.Node, existingAd
 		// for every attachment, go through every network. if the attachment
 		// belongs to one of the networks, then go to the next attachment. if
 		// no network matches, then the the attachment should be removed.
+		if attach.Network == nil {
+			continue
+		}
 		attachmentBelongs := false
 		for _, network := range networks {
 			if network.ID == attach.Network.ID {