We must check if there is an active slave before dereferencing the pointer.

Fixes: 18cb261a ("bonding: support hardware encryption offload to slaves")
Signed-off-by: Nikolay Aleksandrov <razor@blackwall.org>
Reviewed-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>