Fixed the crash caused by improper deletion of VBOs in ParticleSystemQuad

Here is a description of the problem and a simple solution I found. http://www.cocos2d-x.org/boards/6/topics/27145?r=32896
Here is a test project which demonstrates the problem (should be placed in $COCOS2DX_HOME/projects/). https://docs.google.com/file/d/0B1F8RGhitNUxQ3RxdVpDYjZqOUU/edit?usp=sharing