Fix case where Master Volume wasn't used when event had multiple channels
If we get the below event the UPnPClient::VarEventReporter said the Volume was 100 instead of 8.
<Event xmlns="urn:schemas-upnp-org:metadata-1-0/RCS/">
<InstanceID val="0"\>
<Volume channel="Master" val="8"/\>
<Volume channel="LF" val="100"/\>
<Volume channel="RF" val="100"/\>
</InstanceID\>
</Event>
Patch makes sure we get the Master volume back if Event has several optional channels