Add check for existence of 'user_data' in v:completed_item (#258)

The same fix was needed for s:expand().

Author: hokorobi

plugin/vsnip.vim

@@ -115,7 +115,7 @@ function! s:expand() abort
   endfunction
 
   " This is needed to keep normal-mode during 0ms to prevent CompleteDone handling by LSP Client.
-  let l:maybe_complete_done = !empty(v:completed_item) && !empty(v:completed_item.user_data)
+  let l:maybe_complete_done = !empty(v:completed_item) && has_key(v:completed_item, 'user_data') && !empty(v:completed_item.user_data)
   if l:maybe_complete_done
     call timer_start(0, { -> l:ctx.callback() })
   else