On its face, inviting a former grand wizard of the Ku Klux Klan onto your radio show is a risky proposition with very little upside. The situation gets even more precarious when you’re inviting this ex-wizard to dole out opinions on race. But these are wild times we’re living in, which is why David Duke — who has emerged as a top cheerleader for the Republican presidential nominee, Donald J. Trump — appeared on N.P.R.’s “Morning Edition” two weeks ago and defended the candidate from charges of racism.