cli/reply: fix two memory leaks, document a third

cli/reply: fix two memory leaks, document a third

internet_address_list_to_string returns an allocated string, which
needs to be freed with g_free. g_free can handle a NULL argument, so
we follow the usage elsewhere of calling it unconditionally.

The third leak we leave as it would require restructuring of
add_recipients_from_message, and is fixed by later gmime-3.0 porting.