Fix out of range access in Label (#18789)

Out of range occurred in some condition
when using `Label` with its `getLetter()` member function.

`Label::recordPlaceholderInfo()` member function is called for
new line (\n) letter in `Label::multilineTextWrap()`,
but the function doesn't set `_lettersInfo[letterIndex].lineIndex`.

But `lineIndex` is used in `Label::updateLabelLetters()`
even if letter is new line.

This change checks `letterInfo.valid` to avoid
accessing `_linesOffsetX[letterInfo.lineIndex]`
for invalid (i.e. `NewLine`) letter.

cocos/2d/CCLabel.cpp

@@ -766,30 +766,36 @@ void Label::updateLabelLetters()
             else
             {
                 auto& letterInfo = _lettersInfo[letterIndex];
-                auto& letterDef = _fontAtlas->_letterDefinitions[letterInfo.utf32Char];
-                uvRect.size.height = letterDef.height;
-                uvRect.size.width = letterDef.width;
-                uvRect.origin.x = letterDef.U;
-                uvRect.origin.y = letterDef.V;
-
-                auto batchNode = _batchNodes.at(letterDef.textureID);
-                letterSprite->setTextureAtlas(batchNode->getTextureAtlas());
-                letterSprite->setTexture(_fontAtlas->getTexture(letterDef.textureID));
-                if (letterDef.width <= 0.f || letterDef.height <= 0.f)
+                if (letterInfo.valid)
                 {
-                    letterSprite->setTextureAtlas(nullptr);
+                    auto& letterDef = _fontAtlas->_letterDefinitions[letterInfo.utf32Char];
+                    uvRect.size.height = letterDef.height;
+                    uvRect.size.width = letterDef.width;
+                    uvRect.origin.x = letterDef.U;
+                    uvRect.origin.y = letterDef.V;
+
+                    auto batchNode = _batchNodes.at(letterDef.textureID);
+                    letterSprite->setTextureAtlas(batchNode->getTextureAtlas());
+                    letterSprite->setTexture(_fontAtlas->getTexture(letterDef.textureID));
+                    if (letterDef.width <= 0.f || letterDef.height <= 0.f)
+                    {
+                        letterSprite->setTextureAtlas(nullptr);
+                    }
+                    else
+                    {
+                        letterSprite->setTextureRect(uvRect, false, uvRect.size);
+                        letterSprite->setTextureAtlas(_batchNodes.at(letterDef.textureID)->getTextureAtlas());
+                        letterSprite->setAtlasIndex(_lettersInfo[letterIndex].atlasIndex);
+                    }
+
+                    auto px = letterInfo.positionX + letterDef.width / 2 + _linesOffsetX[letterInfo.lineIndex];
+                    auto py = letterInfo.positionY - letterDef.height / 2 + _letterOffsetY;
+                    letterSprite->setPosition(px, py);
                 }
                 else
                 {
-                    letterSprite->setTextureRect(uvRect, false, uvRect.size);
-                    letterSprite->setTextureAtlas(_batchNodes.at(letterDef.textureID)->getTextureAtlas());
-                    letterSprite->setAtlasIndex(_lettersInfo[letterIndex].atlasIndex);
+                    letterSprite->setTextureAtlas(nullptr);
                 }
-
-                auto px = letterInfo.positionX + letterDef.width / 2 + _linesOffsetX[letterInfo.lineIndex];
-                auto py = letterInfo.positionY - letterDef.height / 2 + _letterOffsetY;
-                letterSprite->setPosition(px, py);
-
                 this->updateLetterSpriteScale(letterSprite);
                 ++it;
             }

tests/cpp-tests/Classes/LabelTest/LabelTestNew.cpp

@@ -140,6 +140,7 @@ NewLabelTests::NewLabelTests()
     ADD_TEST_CASE(LabelIssue16717);
     ADD_TEST_CASE(LabelIssueLineGap);
     ADD_TEST_CASE(LabelIssue17902);
+    ADD_TEST_CASE(LabelLetterColorsTest);
 };
 
 LabelFNTColorAndOpacity::LabelFNTColorAndOpacity()
@@ -3567,4 +3568,35 @@ std::string LabelIssue17902::subtitle() const
     return "";
 }
 
+//
+// LabelLetterColorsTest
+//
+LabelLetterColorsTest::LabelLetterColorsTest() {
+    auto center = VisibleRect::center();
 
+    auto label = Label::createWithTTF("", "fonts/arial.ttf", 24);
+    label->setPosition(center.x, center.y);
+    addChild(label);
+
+    label->setString("1\n2\n3");
+    setLetterColors(label, Color3B::RED);
+
+    label->setString("abcd\ne");  // Must not crash at here.
+}
+
+std::string LabelLetterColorsTest::title() const {
+    return "Test for letter colors";
+}
+
+std::string LabelLetterColorsTest::subtitle() const {
+    return "Should not crash!";
+}
+
+void LabelLetterColorsTest::setLetterColors(cocos2d::Label* label, const cocos2d::Color3B& color) {
+    int n = label->getStringLength();
+    for (int i = 0; i < n; ++i) {
+        Sprite* letter = label->getLetter(i);
+        if (letter != nullptr)
+            letter->setColor(color);
+    }
+}

tests/cpp-tests/Classes/LabelTest/LabelTestNew.h

@@ -980,4 +980,17 @@ public:
     virtual std::string subtitle() const override;
 };
 
+class LabelLetterColorsTest : public AtlasDemoNew {
+public:
+    CREATE_FUNC(LabelLetterColorsTest);
+
+    LabelLetterColorsTest();
+
+    virtual std::string title() const override;
+    virtual std::string subtitle() const override;
+
+private:
+    static void setLetterColors(cocos2d::Label* label, const cocos2d::Color3B& color);
+};
+
 #endif