Update index.js

Fixes the case when you pass a bunch of controller files but only some of them contain jsdoc annotations.
eg: 

    app.use(swagger.init(app, {
        apiVersion: '1.0',
        ...,
        apis: fs.readdirSync('./controllers').map(function (filename) {
            return path.join(controllersPath, filename);
        }),
        ...
    }));

lib/swagger-express/index.js

@@ -46,6 +46,11 @@ function parseJsDocs(file, fn) {
     var fragments = js.match(regex);
     var docs = [];
 
+    if (!fragments) {
+      fn(null, docs);
+      return;
+    }
+
     for (var i = 0; i < fragments.length; i++) {
       var fragment = fragments[i];
       var doc = doctrine.parse(fragment, { unwrap: true });