You might think that shooting a movie on the crime-ridden streets of Harlem is asking for trouble, but in the case of New Jack City, you'd be wrong. ''You're not going to have problems with an antidrug movie like ours,'' says director Mario Van Peebles, who nevertheless took the precautionary measure of hiring Louis Farrakhan's Fruit of Islam for security on the production. ''The community knows we're on the right team,'' he adds. Van Peebles worked hard to ensure that New Jack City would be more than just another glamorize-the-gangster film. ''We had to make the balance less Scarface and more Untouchables [with strong good-guy characters]. If you want kids to 'Just Say No' you have to have some role models for them to say yes to.'' The first-time feature-film director didn't have to look far for a role model when he was a kid. His father, Melvin, directed Sweet Sweetback's BaadAsssss Song, one of the top-grossing independent films of 1971. As one would expect, much of Melvin's movie-industry savvy seems to have rubbed off on his son. While making a name for himself as an actor on TV (Sonny Spoon and in such films as Heartbreak Ridge and The Cotton Club, Mario has spent the past seven years quietly building up a directing résumé that includes episodes of 21 Jump Street, Wiseguy, and an ABC Afterschool Special or two. ''I figured if I could show that I could direct white, yellow, and green people, I could define myself as a director before Hollywood defined me as a black director.