Reverted the logic of CCLabelTTF::updateTexture.

In CCLabelTTF::updateTexture function:
The return value of 'CCTexture::initWithString' is different on different platforms.
On android, it will always return true, while on iOS and win32, it will return false if an empty string was passed in.
If we return false for 'updateTexture', the display of LabelTTF will not be updated.
So even if the return value of 'initWithString' is false, we shouldn't return false for 'updateTexture' directly.
It means that we shouldn't check the return value of 'initWithString' here.