The comparisons, Ryan Nembhard understands, are meant as flattery, tinged with hope. Watching dynamic 5-11, 180-pound Nembhard, first in summer league, more recently in preseason games, has caused Mavericks fans to have flashbacks to a pair of other underestimated, smaller-statured Dallas guards. One of them, J.J. Barea, helped lead the Mavericks to the 2011 NBA title. The other, Jalen …