Slightly refactor some GList handling in et_core.c

If calling a g_list_*() method, a pointer to the first element in the
list is always returned, so there is no need to additionally call
g_list_first(). Fix GDestroyNotify functions to return void.
Refactor ET_Add_File_To_Artist_Album_File_List()

Use g_return_val_if_fail() to check invariant.