taglist: check that correct function exists

So 47bf6b1716 added support for
yegappan/taglist plugin, but unfortunately the old (deprecated)
vim-scripts/taglist.vim repo still exists and this one throws an error
when trying to access taglist#Tlist_Get_Tagname_By_Line

So check, that this function is defined before trying to load the
extension

autoload/airline/extensions.vim

@@ -251,6 +251,7 @@ function! airline#extensions#load()
   endif
   if get(g:, 'airline#extensions#taglist#enabled', 1)
         \ && exists(':TlistShowTag')
+        \ && exists('*taglist#Tlist_Get_Tagname_By_Line')
     call airline#extensions#taglist#init(s:ext)
     call add(s:loaded_ext, 'taglist')
   endif

autoload/airline/extensions/taglist.vim

@@ -4,7 +4,7 @@
 
 scriptencoding utf-8
 
-if !exists(':TlistShowTag')
+if !exists(':TlistShowTag') && !exists('*taglist#Tlist_Get_Tagname_By_Line')
   finish
 endif