MINOR: build: Fix build in mux_h1

We want to check if the input buffer contains data, not the connection.
This should unbreak the build.
diff --git a/src/mux_h1.c b/src/mux_h1.c
index c1eefe2..7eaf8cc 100644
--- a/src/mux_h1.c
+++ b/src/mux_h1.c
@@ -2492,7 +2492,7 @@
 	}
 	else {
 		/* If we have a new request, process it immediately */
-		if (unlikely(b_data(&h1c->conn)))
+		if (unlikely(b_data(&h1c->ibuf)))
 			h1_process(h1c);
 		else
 			h1c->conn->xprt->subscribe(h1c->conn, h1c->conn->xprt_ctx, SUB_RETRY_RECV, &h1c->wait_event);