Fix SIGV when grpc_channel_stack_init returns errors

src/core/ext/filters/client_channel/backup_poller.cc

@@ -48,8 +48,8 @@ static gpr_once g_once = GPR_ONCE_INIT;
 static gpr_mu g_poller_mu;
 static backup_poller* g_poller = NULL;  // guarded by g_poller_mu
 // g_poll_interval_ms is set only once at the first time
-// grpc_client_channel_start_backup_polling() is call, after that it is treated
-// as const.
+// grpc_client_channel_start_backup_polling() is called, after that it is
+// treated as const.
 static int g_poll_interval_ms = DEFAULT_POLL_INTERVAL_MS;
 
 static void init_globals() {

src/core/ext/filters/client_channel/client_channel.cc

@@ -713,6 +713,7 @@ static grpc_error *cc_init_channel_elem(grpc_exec_ctx *exec_ctx,
   chand->interested_parties = grpc_pollset_set_create();
   grpc_connectivity_state_init(&chand->state_tracker, GRPC_CHANNEL_IDLE,
                                "client_channel");
+  grpc_client_channel_start_backup_polling(exec_ctx, chand->interested_parties);
   // Record client channel factory.
   const grpc_arg *arg = grpc_channel_args_find(args->channel_args,
                                                GRPC_ARG_CLIENT_CHANNEL_FACTORY);
@@ -754,7 +755,6 @@ static grpc_error *cc_init_channel_elem(grpc_exec_ctx *exec_ctx,
   }
   chand->deadline_checking_enabled =
       grpc_deadline_checking_enabled(args->channel_args);
-  grpc_client_channel_start_backup_polling(exec_ctx, chand->interested_parties);
   return GRPC_ERROR_NONE;
 }