Fix NRE when vessel is destroyed

Don't just remove the CommNode from the net, also kill the vessel's connection field and the PM's comm node reference.  Otherwise it was getting re-added in RACN.Validate.

src/RealAntennasProject/RACommNetVessel.cs

@@ -93,6 +93,8 @@ protected override void OnDestroy()
             GameEvents.onPlanetariumTargetChanged.Remove(OnMapFocusChange);
             base.OnDestroy();
             comm?.Net.Remove(comm);
+            comm = null;
+            vessel.connection = null;
         }
 
         protected override void UpdateComm()