- don't access nonexistent converter goups

- don't try to create formats for converter groups that don't exist
- don't allow playback or recording if there is no corresponding
converter group
- don't try to set audio processing parameters on converter groups
that don't exist

allows playback (or record) only devices to work, such as the
azalias found on ATI graphics devices.

note, the ATI devices generally attach before the onboard devices
do.  that means they will be audio0 and onboard devices will be audio1.
don't forget to make sure the /dev/{audio[ctl],mixer,sound} links
point to the nodes of the device you want to be the default.

tested by naddy@, thanks

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