xdgmimeglob: Fix a memory leak on a duplicate-entry path

Rather than strdup()ing strings when passing them into _xdg_glob_list_append(), strdup() them inside the function instead.

This avoids a leak in the case that the list entry (tuple of data and mime_type) already exists in the list.

Signed-off-by: Philip Withnall pwithnall@gnome.org