Merge pull request #23300 from jiangtaoli2016/alts_envoy

Fix ALTS shutdown crash on envoy

src/core/tsi/alts/handshaker/alts_handshaker_client.cc

@@ -658,12 +658,19 @@ static void handshaker_client_destruct(alts_handshaker_client* c) {
     // TODO(apolcyn): we could remove this indirection and call
     // grpc_call_unref inline if there was an internal variant of
     // grpc_call_unref that didn't need to flush an ExecCtx.
+    if (grpc_core::ExecCtx::Get() == nullptr) {
+      // Unref handshaker call if there is no exec_ctx, e.g., in the case of
+      // Envoy ALTS transport socket.
+      grpc_call_unref(client->call);
+    } else {
+      // Using existing exec_ctx to unref handshaker call.
       grpc_core::ExecCtx::Run(
           DEBUG_LOCATION,
           GRPC_CLOSURE_CREATE(handshaker_call_unref, client->call,
                               grpc_schedule_on_exec_ctx),
           GRPC_ERROR_NONE);
     }
+  }
 }
 
 static const alts_handshaker_client_vtable vtable = {