Don't call release() if CCTexture2D::initWith* fails

Per the discussion on #1952 -- don't call this->release() if CCTexture2D::initWithImage or CCTexture2D::initWithPVRTCData fails.

At a glance, I don't think this would introduce any leaks given that we generally prefer to return a blank/empty CCTexture2D than a NULL pointer (i.e. most code seems to assume that initWithImage etc. will leave the CCTexture2D intact).

That said, it could probably use a quick review :)