closed #2388: Creating a default texture for CCSprite if setTexture(nullptr) is invoked.
issue #2388: Checking whether the texture exists.
issue #2388: One more static_cast.
issue #2388: Updating comments for cc_2x2_white_image variable.
issue #2388: Fixing a bug in FadeIn::clone.
Merge pull request #3114 from dumganhar/iss2388-default-texture

closed #2388: Creating a default texture for CCSprite if setTexture(nullptr) is invoked.