Return from startElement() after parsing <style> node

<style> nodes used to be handled by setting a flag in startElement(),
then processing the node's contents in characters() and resetting the
flag in endElement(). In addition, an item was pushed to m_skipNodes in
startElement() and popped in endElement().

This was simplified by 696599659ebd90753888240573e5c5ca5c90b053. After
that change, QXmlStreamReader::readElementText() was called from
startElement() to process the entire node in one pass. The mistake with
that was that after handling the node, startElement() still proceeded to
push an item to m_skipNodes which wasn't popped anymore because
endElement() was not called for this node anymore. That broke rendering
some files.

This patch here returns from startElement() immediately after processing
the <style> node.

Fixes: QTBUG-150174
Pick-to: 6.12
Change-Id: I50e77f9ac5fc979a1183d260c07a342b7544e5f3
Reviewed-by: Hatem ElKharashy <hatem.elkharashy@qt.io>