When setting up the DPD info we previously would get the
`transport_proto` for the connection with `get_conn_transport_proto`.
This function takes a `conn_id` and would fail fatally if the connection
for the given ID was unknown. It seems it was possible to run into such
scenarios when the `analyzer_violation` event was processed after the
connection had been cleaned up.
We now get the `transport_proto` directly from the ports in the
`connection` passed into `analyzer_violation` via
`get_port_transport_proto` which cannot fail.
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