TBTR: Fix CmdDeleteVirtualTrain deleting non-head vehicles

Deleting a non-head virtual vehicle left a dangling next pointer on
the previous vehicle. Delete the head instead.
Check for non-virtual vehicles

src/vehicle_cmd.cpp

@@ -1109,7 +1109,12 @@ CommandCost CmdDeleteVirtualTrain(TileIndex tile, DoCommandFlag flags, uint32 p1
 		return CMD_ERROR;
 	}
 
+	vehicle = vehicle->First();
+
 	Train* train = Train::From(vehicle);
+	if (!train->IsVirtual()) {
+		return CMD_ERROR;
+	}
 
 	bool should_execute = (flags & DC_EXEC) != 0;