tabline: fix cache invalidation for show_buffers=1 with `:bd`

Ref: https://github.com/bling/vim-airline/pull/700#issuecomment-73798679

autoload/airline/extensions/tabline.vim

@@ -61,9 +61,6 @@ function! s:toggle_on()
   set tabline=%!airline#extensions#tabline#get()
   augroup airline_tabline
     autocmd!
-    " Invalidate cache.
-    autocmd BufAdd,BufUnload * unlet! s:current_buffer_list
-
     if s:buf_min_count <= 0 && s:tab_min_count <= 1
       set showtabline=2
     else
@@ -74,6 +71,10 @@ function! s:toggle_on()
         autocmd TabEnter  * call <sid>show_tabline(s:tab_min_count, tabpagenr('$'))
       endif
     endif
+
+    " Invalidate cache.  This has to come after the BufUnload for
+    " s:show_buffers, to invalidate the cache for BufEnter.
+    autocmd BufAdd,BufUnload * unlet! s:current_buffer_list
   augroup END
 endfunction