Prevent property access on undefined arg.args

Prevents this error message:

    /home/anders/.vim/bundle/tern_for_vim/node_modules/tern/lib/def.js:151
        if (arg instanceof infer.Fn && arg.args.length) addEffect(type, function
                                                ^
    TypeError: Cannot read property 'length' of undefined
        at /home/anders/.vim/bundle/tern_for_vim/node_modules/tern/lib/def.js:151:46