Fix for CCControlButton crashes if getBackgroundSpriteForState is called before init is completed

Extensions
#1

I’m subclassing CCControlButton and need to check background sprites during init, which was crashing because the m_backgroundSpriteDispatchTable had not been setup yet. I added a line to check if it has and return NULL if not.

CCScale9Sprite* CCControlButton::getBackgroundSpriteForState(CCControlState state)
{
    if (m_backgroundSpriteDispatchTable == NULL) return NULL;  // if called before init is completed

    CCScale9Sprite* backgroundSprite = (CCScale9Sprite*)m_backgroundSpriteDispatchTable->objectForKey(state);
    if (backgroundSprite)
    {
        return backgroundSprite;
    }
    return (CCScale9Sprite*)m_backgroundSpriteDispatchTable->objectForKey(CCControlStateNormal);
}

Please add to the source.