tagbar: only try loading the tagbar extension once

commit 232b641 did unfortunately disable tagbar completely, since
exists("*func") does not autoload the function.

So this time, try explicitly calling the function once, and if it does
not exists, it should be disabled and not cause any further errors.

closes #1555

autoload/airline/extensions/tagbar.vim

@@ -9,6 +9,7 @@ endif
 
 let s:flags = get(g:, 'airline#extensions#tagbar#flags', '')
 let s:spc = g:airline_symbols.space
+let s:init=0
 
 " Arguments: current, sort, fname
 function! airline#extensions#tagbar#get_status(...)
@@ -29,6 +30,16 @@ let s:airline_tagbar_last_lookup_time = 0
 let s:airline_tagbar_last_lookup_val = ''
 function! airline#extensions#tagbar#currenttag()
   if get(w:, 'airline_active', 0)
+    if !s:init
+      try
+        " try to load the plugin, if filetypes are disabled,
+        " this will cause an error, so try only once
+        let a=tagbar#currenttag('%', '', '')
+      catch
+      endtry
+      unlet! a
+      let s:init=1
+    endif
     " function tagbar#currenttag does not exist, if filetype is not enabled
     if s:airline_tagbar_last_lookup_time != localtime() && exists("*tagbar#currenttag")
       let s:airline_tagbar_last_lookup_val = tagbar#currenttag('%s', '', s:flags)