selftests: bonding: make sure new active is not null

selftests: bonding: make sure new active is not null

One of Jakub's tests[1] shows that there may be period all ports
are down and no active slave. This makes the new_active_slave null
and the test fails. Add a check to make sure the new active is not null.

 [  189.051966] br0: port 2(s1) entered disabled state
 [  189.317881] bond0: (slave eth1): link status definitely down, disabling slave
 [  189.318487] bond0: (slave eth2): making interface the new active one
 [  190.435430] br0: port 4(s2) entered disabled state
 [  190.773786] bond0: (slave eth0): link status definitely down, disabling slave
 [  190.774204] bond0: (slave eth2): link status definitely down, disabling slave
 [  190.774715] bond0: now running without any active interface!
 [  190.877760] bond0: (slave eth0): link status definitely up
 [  190.878098] bond0: (slave eth0): making interface the new active one
 [  190.878495] bond0: active interface up!
 [  191.802872] br0: port 4(s2) entered blocking state
 [  191.803157] br0: port 4(s2) entered forwarding state
 [  191.813756] bond0: (slave eth2): link status definitely up
 [  192.847095] br0: port 2(s1) entered blocking state
 [  192.847396] br0: port 2(s1) entered forwarding state
 [  192.853740] bond0: (slave eth1): link status definitely up
 # TEST: prio (active-backup ns_ip6_target primary_reselect 1)         [FAIL]
 # Current active slave is null but not eth0

[1] https://netdev-3.bots.linux.dev/vmksft-bonding/results/464481/1-bond-options-sh/stdout

Fixes: 45bf79bc56c4 ("selftests: bonding: reduce garp_test/arp_validate test time")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>