bond: check if slave count is 0 in case when deciding to take slave's mac

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

introduced by:
commit 409cc1f8a41 (bond: have random dev address by default instead of zeroes)

Reported-by: Greg Rose <gregory.v.rose@intel.com>
Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>