Fix crash on route withdrawal with addpath

If addpath is enabled rx then adjRIBIn.RemovePath() will access a member
of p. As p might be nil (e.g. called by fsm_address_family withdraw())
we need to check for this before accessing it.

routingtable/adjRIBIn/adj_rib_in.go

@@ -220,7 +220,7 @@ func (a *AdjRIBIn) removePath(pfx *net.Prefix, p *route.Path) bool {
 	oldPaths := r.Paths()
 	for _, path := range oldPaths {
 		if a.addPathRX {
-			if path.BGPPath.PathIdentifier != p.BGPPath.PathIdentifier {
+			if p != nil && path.BGPPath.PathIdentifier != p.BGPPath.PathIdentifier {
 				continue
 			}
 		}