add missing delete_list() after using list_element.

For memory leaks.
add missing delete_list() after using list_element.

For memory leaks.
Merge branches 'trivial1' and 'trivial1' of https://github.com/iHiroakiKawai/trema-edge into trivial1
Merge pull request #66 from iHiroakiKawai/trivial1

add missing delete_list() after using list_element.