runtime: fix serialization of object lists.

Fixes #500.

artiq/runtime/session.c

@@ -877,7 +877,12 @@ static int send_rpc_value(const char **tag, void **value)
 
         case 'O': { // host object
             struct { uint32_t id; } **object = *value;
-            return out_packet_int32((*object)->id);
+
+            if(!out_packet_int32((*object)->id))
+                return 0;
+
+            *value = (void*)((intptr_t)(*value) + sizeof(*object));
+            break;
         }
 
         default:

artiq/test/coredevice/test_embedding.py

@@ -48,6 +48,9 @@ class RoundtripTest(ExperimentCase):
         obj = object()
         self.assertRoundtrip(obj)
 
+    def test_object_list(self):
+        self.assertRoundtrip([object(), object()])
+
 
 class _DefaultArg(EnvExperiment):
     def build(self):