ppkt: Additional sanity checks.
pcap layer can't free packets once they've been handed off to higher layers.

Those higher layers can queue packets. If pcap frees them after calling
the receive function the higher layer will have a reference to freed
memory. Instead, assume the receive function implies a transfer of
ownership as well.
ppkt: Relax checks on ppkt_cut()

They were a little too strict for the way we use it in ppkt_split().