A Clearwire Corp. shareholder in December launched a putative class action aimed at stopping Sprint Nextel Inc.’s proposed $2.2 billion buyout offer of the broadband Internet service provider, claiming the deal shortchanges investors.  The deal would bring Sprint the nearly 50 percent of Clearwire it does not already own, but the complaint, filed in Delaware Chancery Court by shareholder Abraham Katsman, says that given Sprint’s standing as majority shareholder it must show the transaction is fair to minority investors, a burden it cannot meet.