Mikhail Gorbachev inadvertently stumbled into an old-fashioned capitalist movie squabble during his three-day visit to the United States. But the Soviet leader has received assurances that everything will be OK as long as he returns his videocassette of "Platoon." A Vestron Video spokesman told United Press International that the videocassette was among U. S. souvenirs taken home by the Soviet entourage. The problem is that Vestron is embroiled in a court suit over the rights to distribute the video with Home Box Office Inc. and "Platoon" producer Hemdale Films. In November, a federal district court in California issued a preliminary injunction against HBO, prohibiting it from distributing the tapes until the legal dispute between Vestron and Hemdale had been resolved. Confusion reigns over which company's version of the Academy Award-winning movie Gorbachev got, but Vestron wants his copy returned regardless. Vestron says it will furnish Gorbachev with a copy not included in the judge's ruling.