Once the packet buffer is full on heavy network load, gnrc_netif_hdr_build may return NULL. In that case, the following unchecked access to hdr->data leads to a crash.
At the moment ping is crashing if one pings the loopback address.
This was caused in #8214 when we moved interfaces from `kernel_pid_t`
ID to pointer-based handling, since loopback doesn't evaluate to such
an interface.
