fixed bug exposed by h2_proxy* test.

src/core/lib/channel/http_client_filter.c

@@ -217,7 +217,7 @@ static void hc_mutate_op(grpc_call_element *elem,
     op->recv_initial_metadata_ready = &calld->hc_on_recv;
   }
 
-  if (op->on_complete != NULL) {
+  if (op->on_complete != NULL && op->send_message != NULL) {
     calld->on_complete = op->on_complete;
     op->on_complete = &calld->hc_on_complete;
   }

src/core/lib/channel/http_server_filter.c

@@ -251,11 +251,6 @@ static void hs_mutate_op(grpc_call_element *elem,
         GRPC_MDELEM_CONTENT_TYPE_APPLICATION_SLASH_GRPC);
   }
 
-  if (op->on_complete) {
-    calld->on_complete = op->on_complete;
-    op->on_complete = &calld->hs_on_complete;
-  }
-
   if (op->recv_initial_metadata) {
     /* substitute our callback for the higher callback */
     GPR_ASSERT(op->recv_idempotent_request != NULL);
@@ -271,6 +266,10 @@ static void hs_mutate_op(grpc_call_element *elem,
     if (op->recv_message_ready) {
       op->recv_message_ready = &calld->hs_recv_message_ready;
     }
+    if (op->on_complete) {
+      calld->on_complete = op->on_complete;
+      op->on_complete = &calld->hs_on_complete;
+    }
   }
 }