TWO MORDANT COMEDIAN-ACTORS HAVE PAY-CABLE SPECIALS THIS WEEK. IN HBO's Sandra After Dark (Fri., July 17, midnight) Sandra Bernhard plays the bitchy hostess of a ritzy, risqué Hollywood party, mingling with such guests as Roseanne Arnold, Theresa Russell, Louie Anderson and Timothy Leary. Sandra slinks around in silk pajamas in this twisted take on Hugh Hefner's silly 1969 series Playboy After Dark, as singer Tom Jones and comic Rip Taylor perform, making this soiree sort of an R-rated sleep-over camp. Hardly more conventional is Martin Mull's Talent Takes a Holiday on Showtime (Sat., July 18, 10 PM. ET). It's a cheesy spoof of variety shows from Jackie Gleason's to Garrison Keillor's, with such terrible acts as a Bavarian whistler and an elderly female stand-up comic from Minnesota telling incomprehensible jokes about her husband. Neither of these shows is very funny, but both are terribly strange.