By using a separate variable, first it allows us to sort the lists
of alternates but keep the pointer on the first occurence in the main
playlist (to respect the spec of starting with the bitrate specified
first in the main playlist). It also avoid playing with the lists variable
which should be used to store the list of playlists and not as a pointer
to the current one.
Also fixes a memleak with the g_list_foreach freeing the lists, if it wasn't
pointing to the first element of the list.
