bool cIptvDevice::SetChannelDevice: Support NULL Channel

[MarkusEh]

Please support the NULL Channel in cIptvDevice::SetChannelDevice

See:

https://www.vdr-portal.de/forum/index.php?thread/136284-patch-unbenutzte-frontends-schlie%C3%9Fen/&postID=1371712#post1371712

virtual bool SetChannelDevice(const cChannel *Channel, bool LiveView);
///< Sets the device to the given channel (actual physical setup).
///< If Channel is NULL, this means the device is no longer needed and may
///< turn itself into a power saving mode, if possible. The device must,
///< however, be ready for the next call with a non-NULL Channel at any
///< time.