MINOR: stream-int: factor the SI_ST_EST state test into si_chk_rcv()

This test is made in each implementation of the function, better to
merge it.
diff --git a/include/proto/stream_interface.h b/include/proto/stream_interface.h
index a687b2a..1a70536 100644
--- a/include/proto/stream_interface.h
+++ b/include/proto/stream_interface.h
@@ -396,6 +396,9 @@
 	if (!(si->flags & SI_FL_WANT_PUT))
 		return;
 
+	if (si->state > SI_ST_EST)
+		return;
+
 	si->ops->chk_rcv(si);
 }