Small bugfix.

The worker thread should return NULL, not exit the program.

worker.cc

@@ -100,7 +100,7 @@ static void *worker_thread_start(void *data)
     cudaError_t cudares = cudaSetDevice(cuda_device_id);
     if (cudares != cudaSuccess) {
 	cerr << "Error setting CUDA device: " << cudaGetErrorString(cudares) << "\n";
-	exit(1);
+	return NULL;
     }
 
     bool filled_dp_buffer = false;