MINOR: stream: Pass an optional input buffer when a stream is created

It is now possible to set the buffer used by the channel request buffer when
a stream is created. It may be useful if input data are already received,
instead of waiting the first call to the mux rcv_buf() callback. This change
is mandatory to support H1 connection with no stream attached.

For now, the multiplexers don't pass any buffer. BUF_NULL is thus used to
call stream_create_from_cs().
diff --git a/src/sink.c b/src/sink.c
index a7f6897..35bf770 100644
--- a/src/sink.c
+++ b/src/sink.c
@@ -640,7 +640,7 @@
 		goto out_free_appctx;
 	}
 
-	if ((s = stream_new(sess, &appctx->obj_type)) == NULL) {
+	if ((s = stream_new(sess, &appctx->obj_type, &BUF_NULL)) == NULL) {
 		ha_alert("Failed to initialize stream in peer_session_create().\n");
 		goto out_free_sess;
 	}