lua: fix memory leak in set method

lua: fix memory leak in set method

scan-build from clang version 9 has reported following issue:

 uci.c:624:12: warning: Potential leak of memory pointed to by 's'
       return luaL_error(L, "Cannot set an uci option to an empty table value");
              ^~~~~~~~~~

valgrind confirmed it on the supplied test case:

 ==31013== 8 bytes in 1 blocks are definitely lost in loss record 1 of 1
 ==31013==    by 0x56C49B9: strdup (strdup.c:42)

Signed-off-by: Petr Štetiar <ynezz@true.cz>