Fix QXmlStreamPrivateTagStack uninitialized member

Make sure to init tagsDone from constructor.
Avoid relying on QXmlStreamReaderPrivate::init().

Change-Id: I40c08dd370b0ad519e2d6ebc23b8a906c836508e
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/corelib/serialization/qxmlstream.cpp

@@ -791,6 +791,7 @@ QXmlStreamPrivateTagStack::QXmlStreamPrivateTagStack()
     namespaceDeclaration.prefix = addToStringStorage(u"xml");
     namespaceDeclaration.namespaceUri = addToStringStorage(u"http://www.w3.org/XML/1998/namespace");
     initialTagStackStringStorageSize = tagStackStringStorageSize;
+    tagsDone = false;
 }
 
 #ifndef QT_NO_XMLSTREAMREADER