ao/wasapi: check return values

Only issue a warning for failure of wasapi_enumerate_devices and
wasapi_fill_VistaBlob.
ao/wasapi: more missed cleanup on failure
ao/wasapi: more consistent/reliable method of computing extra WAVEFORMATEXTENSIBLE size
ao/wasapi: remove unnecessary check of audio thread input

it would have caused a deadlock if it fired anyway.
ao/wasapi: handle VistaBlob failure more gracefully