Change name 'av_top' to 'av_top_index'

Change name 'av_top' to 'av_top_index'

 In using the av_top() function created in a recent commit, I found
 myself being confused, and thinking it meant the top element of the
 array, whereas it really means the index of the top element of that
 array.  Since the new name has not appeared in a stable release, it can
 be changed, without remorse, to include 'index' in it.