NFSD fixing possible null pointer derefering in copy offload

Static checker revealed possible error path leading to possible
NULL pointer dereferencing.
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: e0639dc5: ("NFSD introduce async copy feature")
Signed-off-by: Olga Kornievskaia <kolga@netapp.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs4proc.c

@@ -1298,7 +1298,8 @@ nfsd4_copy(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
 out:
 	return status;
 out_err:
-	cleanup_async_copy(async_copy);
+	if (async_copy)
+		cleanup_async_copy(async_copy);
 	goto out;
 }