return if not oplog item

was having error:
```
2014-09-17T23:38:29.251Z - error: TypeError: Cannot read property 'ns' of null
    at /app/server/app.js:229:24
    at /app/node_modules/mongodb/lib/mongodb/cursor.js:257:33
    at /app/node_modules/mongodb/lib/mongodb/cursor.js:758:35
    at Cursor.close (/app/node_modules/mongodb/lib/mongodb/cursor.js:983:5)
    at Cursor.nextObject (/app/node_modules/mongodb/lib/mongodb/cursor.js:758:17)
    at commandHandler (/app/node_modules/mongodb/lib/mongodb/cursor.js:727:14)
    at b (domain.js:183:18)
    at /app/node_modules/mongodb/lib/mongodb/db.js:1893:9
    at b (domain.js:183:18)
    at Server.Base._callHandler (/app/node_modules/mongodb/lib/mongodb/connection/base.js:448:41)
```