BUG/MEDIUM: http: do not dereference strm_li(stream)

Some streams do not have a listener (eg: Lua's cosockets) so
let's check for this. For now this problem cannot happen but
it's definitely unsafe.
diff --git a/src/proto_http.c b/src/proto_http.c
index 0fb57d3..57cc354 100644
--- a/src/proto_http.c
+++ b/src/proto_http.c
@@ -5284,7 +5284,7 @@
 			si_idle_conn(&s->si[1], &srv->idle_conns);
 	}
 
-	s->req.analysers = strm_li(s)->analysers;
+	s->req.analysers = strm_li(s) ? strm_li(s)->analysers : 0;
 	s->res.analysers = 0;
 }