lib: objagg: Fix an error code in objagg_hints_get()

We need to set the error code on this path otherwise we return
ERR_PTR(0) which would result in a NULL dereference in the caller.

Fixes: 9069a381 ("lib: objagg: implement optimization hints assembly and use hints for object creation")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>