[pvr] fix unnecessary creation of new CPVRChannel instances

Has seen during integration of Radio RDS in XBMC the music player info generate always the CPVRChannel class new if not present, with this e.g. 
- CPVRChannel* channel = m_currentFile->GetPVRChannelInfoTag();
- return channel ? channel->FormattedChannelNumber() : "";
has always then returned a new class "0" string without PVR.

Used this to identify PVR channel on music OSD skin with 
<visible>Player.HasAudio + !IsEmpty(MusicPlayer.ChannelNumberLabel)</visible>
and found it.