`od_hex_dump()` is called if `hdr_len < pkt->size` to print the rest
after `hdr_len` of `pkt`. So if we just leave `hdr_len = 0` instead of
calling `od_hex_dump()` for every other NETTYPE, we achieve the same
effect.
As it is more effective (and already done in some cases) to re-set
`hdr_len` when the header was printed, we initialize `hdr_len` first
with 0 now.
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