Kellogg, the maker of Corn Flakes and Rice Krispies, reported a bigger-than-expected drop in quarterly net sales as demand for its snacks and cereals remained weak in the United States. Sales in the company's U.S. snacks business — its biggest —fell 1.5 percent, while sales in the U.S. morning foods business, which includes cereals, fell 2.6 percent. Sales in both businesses fell for the third straight quarter.