The Dolphins took a huge risk on selecting the brittle virtuoso Tua Tagovailoa, but if he can stay upright, the Dolphins could be a big winner. They kept fortifying the edge of the offensive line with Austin Jackson, and then, after adding a fourth-round selection in a trade down with the Green Bay Packers, finished Thursday with Auburn cornerback Noah Igbinoghene. Are they just building up depth, or is there any concern at all about Xavien Howard, who has missed at least a quarter of the season in three of his four years and lined up for only 40 of the team’s 65 games in that span?