Fix memory leak in QDomDocument DTD notation declaration handler

The created notation node's reference count needs to be decremented to 0 before it is
added as a child, because appendChild will increment the reference count to correct
value of 1. Also added autotest DTDNotationDecl to tst_qdom to expose the leak when
executed under valgrind memcheck. There was no previous test coverage for the notation
declarations in DTD.

Task-number: QTBUG-22588
Change-Id: I452d45cb70dfcda48061e0d4af5085ab1c1ad59e
(From Qt5 commit 1bc31fa43ddefe468c9f079156bfad0371e2a61b)
Reviewed-by: Olivier Goffart <[email protected]>

Author: Sami Rosendahl

src/xml/dom/qdom.cpp

@@ -7557,6 +7557,8 @@ bool QDomHandler::externalEntityDecl(const QString &name, const QString &publicI
 bool QDomHandler::notationDecl(const QString & name, const QString & publicId, const QString & systemId)
 {
     QDomNotationPrivate* n = new QDomNotationPrivate(doc, 0, name, publicId, systemId);
+    // keep the refcount balanced: appendChild() does a ref anyway.
+    n->ref.deref();
     doc->doctype()->appendChild(n);
     return true;
 }

tests/auto/qdom/tst_qdom.cpp

@@ -132,6 +132,7 @@ private slots:
 
     void taskQTBUG4595_dontAssertWhenDocumentSpecifiesUnknownEncoding() const;
     void cloneDTD_QTBUG8398() const;
+    void DTDNotationDecl();
 
     void cleanupTestCase() const;
 
@@ -1930,5 +1931,28 @@ void tst_QDom::cloneDTD_QTBUG8398() const
     domDocument2.save(stream, 0);
     QCOMPARE(output, expected);
 }
+
+void tst_QDom::DTDNotationDecl()
+{
+    QString dtd("<?xml version='1.0' encoding='UTF-8'?>\n"
+                   "<!DOCTYPE first [\n"
+                   "<!NOTATION gif SYSTEM 'image/gif'>\n"
+                   "<!NOTATION jpeg SYSTEM 'image/jpeg'>\n"
+                   "]>\n"
+                   "<first/>\n");
+
+    QDomDocument domDocument;
+    QVERIFY(domDocument.setContent(dtd));
+
+    const QDomDocumentType doctype = domDocument.doctype();
+    QCOMPARE(doctype.notations().size(), 2);
+
+    QVERIFY(doctype.namedItem(QString("gif")).isNotation());
+    QCOMPARE(doctype.namedItem(QString("gif")).toNotation().systemId(), QString("image/gif"));
+
+    QVERIFY(doctype.namedItem(QString("jpeg")).isNotation());
+    QCOMPARE(doctype.namedItem(QString("jpeg")).toNotation().systemId(), QString("image/jpeg"));
+}
+
 QTEST_MAIN(tst_QDom)
 #include "tst_qdom.moc"