QSvgHandler: Copy xml->name() when passing it to startElement()

It is just a QStringView to QXmlStreamReader's internal data and that
may be freed when using the reader, resulting in dangling pointers.

Since 696599659ebd90753888240573e5c5ca5c90b053,
QXmlStreamReader::readNext() is called from startElement(). At the
moment, the QStringView localName isn't used anymore after that call but
that might change accidentally. Copying the data to a QString avoids
this risk.

Pick-to: 6.12
Change-Id: If676504d333ea447b61bffbe83fb10fbd9a4d4bd
Reviewed-by: Hatem ElKharashy <hatem.elkharashy@qt.io>