Fix memory leak if invoking Texture2D::setAlphaTexture many times. (#19400)

Author: dumganhar

cocos/renderer/CCTexture2D.cpp

@@ -1499,10 +1499,12 @@ void Texture2D::removeSpriteFrameCapInset(SpriteFrame* spriteFrame)
 /// halx99 spec, ANDROID ETC1 ALPHA supports.
 void Texture2D::setAlphaTexture(Texture2D* alphaTexture)
 {
-    if (alphaTexture != nullptr) {
-        this->_alphaTexture = alphaTexture;
-        this->_alphaTexture->retain();
-        this->_hasPremultipliedAlpha = true; // PremultipliedAlpha should be true.
+    if (alphaTexture != nullptr)
+    {
+        alphaTexture->retain();
+        CC_SAFE_RELEASE(_alphaTexture);
+        _alphaTexture = alphaTexture;
+        _hasPremultipliedAlpha = true; // PremultipliedAlpha should be true.
     }
 }