upstream: add null check for stream cleanup

Author: zhihonl

src/flb_upstream.c

@@ -680,7 +680,11 @@ int flb_upstream_destroy(struct flb_upstream *u)
     flb_free(u->proxied_host);
     flb_free(u->proxy_username);
     flb_free(u->proxy_password);
-    mk_list_del(&u->base._head);
+
+    if (mk_list_is_set(&u->base._head) == 0) {
+        mk_list_del(&u->base._head);
+    }
+    
     flb_free(u);
 
     return 0;