autocommands: check for existence of airline highlight group

Several extensions trigger custom actions. Make sure, those actions are
only performed, if airline is actually active and not temporarily
disabled using e.g. :AirlineToggle

autoload/airline/extensions/ale.vim

@@ -128,6 +128,10 @@ function! airline#extensions#ale#init(ext)
 endfunction
 
 function! s:ale_refresh()
+  if !exists('#airline')
+    " airline disabled
+    return
+  endif
   if get(g:, 'airline_skip_empty_sections', 0)
     exe ':AirlineRefresh!'
   endif