Label::getLetter lose label's color

[voidartworks]

• cocos2d-x version: 3.12
• devices test on: iOS 11.3/iPad Air 2
• developing environments
  • Xcode version: 9.3

Steps to Reproduce:

`Label* pLabel = Label::createWithTTF("TEST", LOCALIZED_FONT, 60);
pLabel->setPosition(WIN_SIZE.width / 2, WIN_SIZE.height / 2);
addChild(pLabel);

pLabel->setColor(Color3B::RED);
pLabel->getLetter(0);`

It wil show a label with white "T" and red "EST".