vlcmediawidget: initialize urlIsAudioCd and trackNumber variables

Avoids this valgrind warning:

==7224== Conditional jump or move depends on uninitialised value(s)
==7224== at 0x4D071F: VlcMediaWidget::updateMetadata() (vlcmediawidget.cpp:635)

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>