Merge pull request #15978 from yang-g/hanging_transport

Do not start timer if transport is closed.

src/core/ext/transport/chttp2/transport/chttp2_transport.cc

@@ -2663,6 +2663,9 @@ static void init_keepalive_ping_locked(void* arg, grpc_error* error) {
 
 static void start_keepalive_ping_locked(void* arg, grpc_error* error) {
   grpc_chttp2_transport* t = static_cast<grpc_chttp2_transport*>(arg);
+  if (error != GRPC_ERROR_NONE) {
+    return;
+  }
   GRPC_CHTTP2_REF_TRANSPORT(t, "keepalive watchdog");
   grpc_timer_init(&t->keepalive_watchdog_timer,
                   grpc_core::ExecCtx::Get()->Now() + t->keepalive_timeout,