BUG/MINOR: mux-h1: Release idle server H1 connection if data are received

When data are received on an idle H1 connection on server side, we must take
care to release the connection. Most of time, it will be a
408-Request-Time-out response. But, in all cases, these data should never be
processed as a response to a client.

There is no upstream ID because the 2.4 is not affected by this bug. It must
be backported as far as 2.0.
diff --git a/src/mux_h1.c b/src/mux_h1.c
index d4ea17d..c55f6c4 100644
--- a/src/mux_h1.c
+++ b/src/mux_h1.c
@@ -2246,6 +2246,8 @@
 			if (!h1s_create(h1c, NULL, NULL))
 				goto release;
 		}
+		else if (conn_is_back(conn) && (h1c->flags & H1C_F_CS_IDLE) && b_data(&h1c->ibuf))
+			goto release;
 		else
 			goto end;
 		h1s = h1c->h1s;