Add secondary check for listed buffers when closing.

If BufExplorer's MRUList is empty or has no listed buffers in it, create
an array of listed buffers from vim's buffer list instead.

plugin/bufexplorer.vim

@@ -914,6 +914,9 @@ endfunction
 function! s:Close()
     " Get only the listed buffers.
     let listed = filter(copy(s:MRUList), "buflisted(v:val)")
+    if len(listed) == 0
+        let listed = filter(range(1, bufnr('$')), "buflisted(v:val)")
+    endif
 
     " If we needed to split the main window, close the split one.
     if s:splitMode != "" && bufwinnr(s:originBuffer) != -1