clear self.std* to avoid writing to empty pipes

This ensures that behavior between a newly opened nvim and one in which tabnine-chat has been opened and then closed are identical

lua/tabnine/chat/binary.lua

@@ -24,6 +24,9 @@ function ChatBinary:close()
 		uv.kill(self.pid, "sigterm")
 		self.handle = nil
 		self.pid = nil
+		self.stdin = nil
+		self.stdout = nil
+		self.stderr = nil
 	end
 end