[src] fix cudadecoder crash with invalid --cuda-worker-threads (kaldi-asr#4596)

num_worker_threads had been pulled from the `config` struct,
not `config_`. The latter is sanitized by a call to
`CheckAndFixConfigs()`, the former isn't. This was the root
cause for the segfault.

Author: quadr

src/cudadecoder/batched-threaded-nnet3-cuda-online-pipeline.h

@@ -137,7 +137,7 @@ class BatchedThreadedNnet3CudaOnlinePipeline {
     config_.compute_opts.CheckAndFixConfigs(am_nnet_->GetNnet().Modulus());
     config_.CheckAndFixConfigs();
     Initialize(decode_fst);
-    int num_worker_threads = config.num_worker_threads;
+    int num_worker_threads = config_.num_worker_threads;
     thread_pool_ = std::make_unique<ThreadPoolLight>(num_worker_threads);
   }