Add multiple free for each call to lookup_args()

Add multiple free for each call to lookup_args()

This memory leak can be reproduced using a tight loop to uci:get() a config.

lua -e 'require("posix"); require("uci");
function getconf()
   local tmp, uc = uci.cursor();
   tmp = uc:get("system", "ntp", "enable_server");
end;

while 1
do
   for i=0,1000 do getconf() end;
   print("gc");
   print(collectgarbage("count"));
   collectgarbage();
   print(collectgarbage("count"));
   print("sleep 1");
   posix.sleep(1);
end'