Codechange: add non-nullptr asserts in cases where it should never be nullptr

Though where similar calls are checked for nullptr as in those instances of
the use of that function it can actually return nullptr. In other words, write
down the assumption that the function never returns nullptr in an assert.

src/order_cmd.cpp

@@ -1251,6 +1251,7 @@ CommandCost CmdModifyOrder(DoCommandFlag flags, VehicleID veh, VehicleOrderID se
 	if (sel_ord >= v->GetNumOrders()) return CMD_ERROR;
 
 	Order *order = v->GetOrder(sel_ord);
+	assert(order != nullptr);
 	switch (order->GetType()) {
 		case OT_GOTO_STATION:
 			if (mof != MOF_NON_STOP && mof != MOF_STOP_LOCATION && mof != MOF_UNLOAD && mof != MOF_LOAD) return CMD_ERROR;