Name the tag when a BTags entry is too short to hold an address

A source of fewer than three columns raised E684 from parts[2], which the loop
reported in place of the message the line below it gives.

autoload/fzf/vim.vim

@@ -1712,9 +1712,9 @@ function! s:btags_sink(from, lines)
     for line in a:lines[1:]
       try
         let parts = split(line, "\t")
-        let excmd = s:tag_address(parts[2], 0)
+        let excmd = len(parts) > 2 ? s:tag_address(parts[2], 0) : ''
         if empty(excmd)
-          throw 'Unsupported tag address: '.s:strip(parts[0])
+          throw 'Unsupported tag address: '.s:strip(get(parts, 0, line))
         endif
         call s:execute_tag_address(excmd)
         call add(qfl, {'filename': expand('%'), 'lnum': line('.'), 'text': getline('.')})