fix a memory bug in CudnnHolder

paddle/fluid/platform/device_context.cc

@@ -176,13 +176,12 @@ class CudnnHolder {
     if (required_workspace_len <= workspace_len_) {
       return;
     }
-    void* new_workspace = paddle::memory::Alloc(place_, required_workspace_len);
     if (workspace_ != nullptr) {
       // Maybe someone is using the current workspace
       PADDLE_ENFORCE(cudaStreamSynchronize(*stream_));
       paddle::memory::Free(place_, workspace_);
     }
-    workspace_ = new_workspace;
+    workspace_ = paddle::memory::Alloc(place_, required_workspace_len);
     workspace_len_ = required_workspace_len;
   }