fixes#1187 by making sure, we only call functions, that actually exist.
Should make vim-airline work with lady loaded YCM, e.g. using vim-plug:
```viml
" Code to execute when the plugin is loaded on demand
Plug 'Valloric/YouCompleteMe', { 'for': 'cpp' }
autocmd! User YouCompleteMe if !has('vim_starting') | call youcompleteme#Enable() | endif
```
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