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/mux_h2.c b/src/mux_h2.c
index 741708d..75874a3 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -1483,7 +1483,7 @@
 	cs->ctx = h2s;
 	h2c->nb_cs++;
 
-	if (stream_create_from_cs(cs) < 0)
+	if (stream_create_from_cs(cs, &BUF_NULL) < 0)
 		goto out_free_cs;
 
 	/* We want the accept date presented to the next stream to be the one