The economy, held back by the summer drought, expanded at a moderate annual rate of 2.5 percent last summer, the slowest pace in nearly two years, the government said Tuesday. The Commerce Department said the gross national product, the broadest measure of economic health, slowed in the third quarter after growing at a robust 3.4 percent pace in the first quarter and 3.0 percent in the second. That would be good news for analysts worried that the economy is overheating and becoming inflationary, except that the underlying strength is being masked by the temporary effects of the drought, which seared pastures and shriveled crops.