batman-adv: bla: avoid NULL-ptr deref for claim via dropped interface

In the Linux kernel, the following vulnerability has been resolved: batman-adv: bla: avoid NULL-ptr deref for claim via dropped interface Without rtnl_lock held, a hardif might be retrieved as primary interface of a meshif, but then (while operating on this interface) getting decoupled from the mesh interface. In this case, the meshif still exists but the pointer from the primary hardif to the meshif is set to NULL. The mesh_iface must be checked first to be non-NULL before continuing to send an ARP request using meshif.