in bond_enslave(), check slave_cnt before actually using slave address.

introduced by:
commit 409cc1f8 (bond: have random dev address by default instead of zeroes)
Reported-by: NGreg Rose <gregory.v.rose@intel.com>
Signed-off-by: NJiri Pirko <jiri@resnulli.us>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>