They are related in that they both originate from 'niger', the Latin for black. The abusive word n*gger is a corruption of 'negro', meaning a black person (negro means Black in Spanish, a language derived from Latin). The River Niger in Africa (third largest after the Nile and the Congo) gets its name from the Latin for black, though it's uncertain whether it refers to the appearance of the river or to the native peoples.
unless you are referring to 'your *******' in a friendly term. It is a common word used among the young black community and it depends who you are and how you say it as to how offensive it is.