[cocos2dx] PolygonBatch::flush crash fix

Same fix as in https://github.com/EsotericSoftware/spine-runtimes/pull/224/files but for cocos2dx
Referenced here http://forum.cocos2d-swift.org/t/spine-runtime-for-3-0-problems/13504

"bind VAO 0 to clear the current vao context"