batman-adv: Don't schedule OGM for disabled interface

A transmission scheduling for an interface which is currently dropped by
batadv_iv_ogm_iface_disable could still be in progress. The B.A.T.M.A.N. V
is simply cancelling the workqueue item in an synchronous way but this is
not possible with B.A.T.M.A.N. IV because the OGM submissions are
intertwined.

Instead it has to stop submitting the OGM when it detect that the buffer
pointer is set to NULL.

Reported-by:  <syzbot+a98f2016f40b9cd3818a@syzkaller.appspotmail.com>
Reported-by:  <syzbot+ac36b6a33c28a491e929@syzkaller.appspotmail.com>
Fixes: c6c8fea2

 ("net: Add batman-adv meshing protocol")
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Cc: Hillf Danton <hdanton@sina.com>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>