BUG/MINOR: promex: Don't forget to consume the request on error

When the promex applet triggers an error, for instance because the URI is
invalid, we must still take care to consume the request. Otherwise, the
error will be handled by HTTP analyzers as a server abort.

This patch must be backported as far as 2.0.

addons/promex/service-prometheus.c

@@ -1617,6 +1617,7 @@ static void promex_appctx_handle_io(struct appctx *appctx)
 	res->flags |= CF_READ_EVENT;
 	sc_shutr(sc);
 	sc_shutw(sc);
+	goto out;
 }
 
 struct applet promex_applet = {