extensions: prevent error about unknown function for ycm

if g:loaded_youcompleteme does not exists, the ycm extension will not be
loaded. However extensions.vim could still try to init the ycm
extension, which would lead to an error message:

  E117: unknown function airline#extensions#ycm#init

so make sure, it is only loaded, if ycm is loaded as well.

autoload/airline/extensions.vim

@@ -176,7 +176,7 @@ function! airline#extensions#load()
     call add(s:loaded_ext, 'term')
   endif
 
-  if get(g:, 'airline#extensions#ycm#enabled', 0)
+  if get(g:, 'airline#extensions#ycm#enabled', 0) && exists('g:loaded_youcompleteme')
     call airline#extensions#ycm#init(s:ext)
     call add(s:loaded_ext, 'ycm')
   endif