Staging: intel_sst: fix memory leak

The original code set "str_info->decode_ibuf" to NULL so the kfree() is
no-op.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Harsha Priya <priya.harsha@intel.com>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>