LOS ANGELES — Nick Reiner, the 32-year-old son of acclaimed director Rob Reiner, pleaded not guilty Monday to two counts of first-degree murder in the fatal stabbing of his parents in December. Reiner’s attorney, Deputy Public Defender Kimberly Greene, entered the plea on his behalf as he stood behind glass in an enclosed custody area of the packed Los Angeles courtroom. The third of Rob Reiner’s four children, Nick Reiner has been held without bail since his arrest hours after the actor-director and Michele Singer Reiner, his wife of 36 years, were found dead on Dec. 14 at their home in the upscale Brentwood section of Los Angeles. Reiner appeared in court with his head shaved and wearing brown jail clothes. He talked to his lawyer briefly before the judge began the hearing and spoke only to answer yes to a question from the judge. He was not wearing the suicide prevention smock he wore in his first court appearance in December. The judge told Reiner to return to court April 29 for the scheduling of a preliminary hearing where prosecutors will present evidence and a judge will dec