noting that a defendant waives an affirmative defense by failing to request a jury instruction on it | affirming a substantial upward variance based on extraordinary circumstances of cocaine distribution offense, which included defendant’s distribution of cocaine to a minor and sexual assault | explaining that, while the district court on remand could not have applied an upward departure based on conduct we previously deemed beyond the scope of relevant conduct under section 1B1.3, it could impose a variance based on such conduct under section 3553(a)