[CoreAudioAE] Fix CFString copy function in CCoreAudioHardware::GetOutputDeviceName

CFStringGetCStringPtr fails "often" (see documentation on the issue) and returns
NULL. Since this code path is not critical of speed we can use the "safer"
CFStringGetCString copy functions instead and be sure that we get a proper value.

This manifested in that AirPlay output device (or whatever was the "first" device)
was always selected on a clean start.