ALSA: compress_core: cleanup pointers on stop

as the start can be called after stop again, we need to reset state

Signed-off-by: Namarta Kohli <namartax.kohli@intel.com>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>