Weakened revenue and an increase in store closures by Barnes & Noble Inc. calls the bookseller’s stability into question. Agha Nawazish Ali Khan, a securities analyst for SNL Financial, a San Francisco financial services firm, flags Barnes & Noble’s disappointing holiday sales in a report highlighting its role as an important tenant for some of the country’s top mall operators. Barnes & Noble (NYSE: BKS) operates seven stores in Oregon, including four in the Portland area and one each in…