Fix crash / debug assertion failure from explosions (#535)

When explosions are spawned they grab all entities in range and interact with them. They don't check if entities are deleted before doing so which can cause a crash. 

To reproduce, place several grenades on the ground, pick one up and detonate it on top of the others so they detonate as well. The debug assertion in ContainerHelpers.IsInContainer will fail due to one of the accessed entities having been deleted.

This fixes that by ignoring deleted entities when making explosions.

Content.Server/Explosions/ExplosionHelper.cs

@@ -34,10 +34,11 @@ namespace Content.Server.Explosions
 
             foreach (var entity in entitiesAll)
             {
-                //if (entity == Owner)
-                //    continue;
+                if (entity.Deleted)
+                    continue;
                 if (!entity.Transform.IsMapTransform)
                     continue;
+
                 var distanceFromEntity = (int)entity.Transform.GridPosition.Distance(mapManager, coords);
                 var exAct = entitySystemManager.GetEntitySystem<ActSystem>();
                 var severity = ExplosionSeverity.Destruction;