bond: Handle unknown slaves in bond_check_admissibility().

In future patches, ofproto-dpif-xlate may be temporarily out of
sync with ofproto-dpif and pass a non-bonded ofport into
bond_check_admissibility().  This patch handles that edge case
gracefully.

Signed-off-by: Ethan Jackson <ethan@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>