Fix check for vim9 script

closes #2636

autoload/airline/util.vim

@@ -233,6 +233,7 @@ endfunction
 function! airline#util#has_vim9_script()
   " Returns true, if Vim is new enough to understand vim9 script
   return (exists(":def") &&
+    \ exists("v:versionlong") &&
     \ v:versionlong >= 8022844 &&
     \ get(g:, "airline_experimental", 0))
 endfunction