agent: Add missing stream-closed checks for pseudo-TCP
This got lost in rebasing the FIN–ACK work. component->tcp is now only NULL before the pseudo-TCP stream is initially created — afterwards, it is always non-NULL, but pseudo_tcp_stream_is_closed() returns TRUE if the stream has closed (gracefully or on errors). This allows differentiation between the states: TCP support was never initialised; and TCP support was initialised but is now closed.
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