Unlet s:last_message with no error in show_list (#329)

If entries is empty, it will try to unlet of non-existing variable and throw the error.

References #328

autoload/startify.vim

@@ -468,7 +468,7 @@ function! s:show_lists(lists) abort
         continue
       endtry
       if empty(entries)
-        unlet s:last_message
+        unlet! s:last_message
         continue
       endif