Fix a double deletion in QDomAttr::setNodeValue()

Check the reference count before deleting. Patch
as contributed on bug report.

Pick-to: 5.15 5.12
Fixes: QTBUG-86547
Change-Id: I2cb197e3eeda7ade2442c23f6b4f1ae6ff2ff810
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/xml/dom/qdom.cpp

@@ -3625,7 +3625,9 @@ void QDomAttrPrivate::setNodeValue(const QString& v)
     // keep the refcount balanced: appendChild() does a ref anyway.
     t->ref.deref();
     if (first) {
-        delete removeChild(first);
+        auto removed = removeChild(first);
+        if (removed && !removed->ref)
+            delete removed;
     }
     appendChild(t);
 }