Fix lighting errors when all lights are disabled.

When scene has zero lights sources actives, last light buffer are used for render. This cause render artifatcs.

![screenshot 2014-12-03 23 48 13](https://cloud.githubusercontent.com/assets/2388307/5290412/e0372976-7b46-11e4-8556-19bbb8190648.png)

Thx @Lopuska for the fix :)