Napoli and Juventus continued their struggles in this season's Champions League on Tuesday when they were held to draws at home by Eintracht Frankfurt and Sporting Lisbon.     Serie A champions Napoli have four points from four games after being held to a dull goalless draw.     The Turin giants have one point less at the league stage's half-way point, after Dusan Vlahovic cancelled out Maximiliano Araujo's opener for the Portuguese visitors to earn them a 1-1 draw in their first European game