discoverer: Don't crash when running with -v if channel-mask==0 and >7 channels

For e.g. 16-channel audio, if the channel mask is 0 (which it usually
is), gst_audio_channel_positions_from_mask would get confused,
ultimately leading into a crash.

https://bugzilla.gnome.org/show_bug.cgi?id=796578
1 file changed