staging/goldfish/goldfish_audio: fix a sparse warning

Fix a pointer check to use NULL instead of 0

Warning:
drivers/staging/goldfish/goldfish_audio.c:300:34: warning: Using plain
integer as NULL pointer

Signed-off-by: Robin Schroer <sulamiification@gmail.com>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>