smb3: fix potential null dereference in decrypt offload

commit a091c5f67c99 ("smb3: allow parallelizing decryption of reads")
had a potential null dereference

Reported-by: kbuild test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Suggested-by: Pavel Shilovsky <pshilov@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>