Added mixer channel count check to prevent -10877 errors.

I was running into similar issues as Pascal was on the forum (see: http://forum.theamazingaudioengine.com/discussion/312/limited-number-of-channels-filters-or-groups-solved/p1) - where `AEAudioController -createChannelGroupWithinChannelGroup:` was causing errors like the following to appear:

    AEAudioController.m:2905: AudioUnitSetParameter(kMultiChannelMixerParam_Volume) result -10877 FFFFD583 ˇˇ’É
    AEAudioController.m:2912: AudioUnitSetParameter(kMultiChannelMixerParam_Pan) result -10877 FFFFD583 ˇˇ’É
    AEAudioController.m:2917: AudioUnitSetParameter(kMultiChannelMixerParam_Enable) result -10877 FFFFD583 ˇˇ’É
    AEAudioController.m:1137: Update graph result -10877 FFFFD583 ˇˇ’É

This PR solves that issue by ensuring the mixer with channels being added to it has enough input busses to handle the new channel.