in NULL reference for incoming TCP packets". Loopif has to be configured
(using LWIP_LOOPIF_MULTITHREADING) to directly call netif->input()
(multithreading environments, e.g. netif->input() = tcpip_input()) or
putting packets on a list that is fed to the stack by calling loopif_poll()
(single-thread / NO_SYS / polling environment where e.g.
netif->input() = ip_input).