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

plugin/airline.vim

@@ -230,7 +230,7 @@ function! s:airline_refresh(...)
 endfunction
 
 function! s:FocusGainedHandler(timer)
-  if exists("s:timer") && a:timer == s:timer
+  if exists("s:timer") && a:timer == s:timer && exists('#airline')
     augroup airline
       au FocusGained * call s:on_focus_gained()
     augroup END