Break after WriteLast() (#27165)

If we continue to call `Write()` after `WriteLast()` (because client is still sending us data and thus `Read()` returns `true`), the buffer might become invalid and results in assertion failures.

test/cpp/end2end/test_service_impl.h

@@ -403,6 +403,7 @@ class TestMultipleServiceImpl : public RpcService {
       response.set_message(request.message());
       if (read_counts == server_write_last) {
         stream->WriteLast(response, WriteOptions());
+        break;
       } else {
         stream->Write(response);
       }