Staging: batman-adv: Don't dereference unchecked incoming soft_iface

send_packet derefenced forw_packet->if_incoming and checked if
forw_packet->if_incoming is NULL.

This cannot happen, but still makes irritates when reading through the
functions.

Reported-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Sven Eckelmann <sven.eckelmann@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>