apparmor: make sure the decompression ctx is promperly initialized

The decompress ctx was not properly initialized when reading raw
profile data back to userspace.

Reported-by: kernel test robot <lkp@intel.com>
Fixes: 52ccc20c652b ("apparmor: use zstd compression for profile data")
Signed-off-by: John Johansen <john.johansen@canonical.com>