QXmlStreamReader::readNextStartElement() - return false on document end

The method reads the next element in a loop, as long as valid elements
exist. Within the loop, it returns
- false if the end of an element has been reached
- true if a new element has started

When the document end has been reached, the loop continues, until
readNext() returns Invalid. Then, PrematureEndOfDocumentError is launched.

This is wrong, because reading beyond the document end is caused by a
missing return condition in the loop.

=> Treat document end like element end and return false without
reading beyond it.

=> Test correct behavior in tst_QXmlStream::readNextStartElement()

Fixes: QTBUG-25944
Change-Id: I0160b65880756a2be541e9f55dc79557fcb1f09f
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit ccd8a496cf46313ea0df9fe70f4e4b57f9434b24)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/serialization/qxmlstream.cpp

@@ -721,7 +721,7 @@ QXmlStreamReader::TokenType QXmlStreamReader::tokenType() const
 bool QXmlStreamReader::readNextStartElement()
 {
     while (readNext() != Invalid) {
-        if (isEndElement())
+        if (isEndElement() || isEndDocument())
             return false;
         else if (isStartElement())
             return true;

tests/auto/corelib/serialization/qxmlstream/tst_qxmlstream.cpp

@@ -1147,6 +1147,10 @@ void tst_QXmlStream::readNextStartElement() const
     }
 
     QCOMPARE(amountOfB, 2);
+
+    // well-formed document end follows
+    QVERIFY(!reader.readNextStartElement());
+    QCOMPARE(reader.error(), QXmlStreamReader::NoError);
 }
 
 void tst_QXmlStream::readElementText() const