MINOR: tcp-rules: check that the listener exists before updating its counters

MINOR: tcp-rules: check that the listener exists before updating its counters

The tcp rules may be applied to a TCP stream initiated by applets (spoe,
lua, peers, later H2). These ones do not necessarily have a valid listener
so we must verify the field is not null before updating the stats. For now
there's no way to trigger this bug because lua and peers don't have analysers,
h2 is not implemented and spoe has a dummy listener. But this threatens to
break at any instant.