aout: fix aout_DevicesList() error handling
aout: fix aout_DevicesList() error handling

Return -1 if and only if there was an error. Never return an incomplete
list, and do not pretend an error occurred when the devices list is
merely empty.

xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc