IVF-PQ tests: fix segfault when accessing empty lists (#838)

Check if the `index.lists()[label]` is not an empty shared pointer before accessing its content in tests.

If the list size is zero, the list shared pointer can be `.reset()` during deserialization/resizing. Hence IVF-Flat and IVF-PQ can sometimes have empty shared pointers in their `.lists()` members. In case of IVF-PQ, we've had a piece of tests where this wasn't checked before accessing. As a result, we have been observing occasional CI failures with segfaults - when the IVF-PQ index was very unbalanced, i.e. had empty lists.

So far, the affected test is the only place I could identify where the access to the list pointer is unprotected (either via explicit check or by the invariant list_size > 0).

Authors:
  - Artem M. Chirkin (https://github.com/achirkin)
  - Corey J. Nolet (https://github.com/cjnolet)

Approvers:
  - Bradley Dice (https://github.com/bdice)
  - Tamas Bela Feher (https://github.com/tfeher)

URL: #838

Author: achirkin

cpp/tests/neighbors/ann_ivf_pq.cuh

@@ -283,9 +283,11 @@ class ivf_pq_test : public ::testing::TestWithParam<ivf_pq_inputs> {
     // the original data cannot be reconstructed since the dataset was normalized
     if (index.metric() == cuvs::distance::DistanceType::CosineExpanded) { return; }
     auto& rec_list = index.lists()[label];
-    auto dim       = index.dim();
-    n_take         = std::min<uint32_t>(n_take, rec_list->size.load());
-    n_skip         = std::min<uint32_t>(n_skip, rec_list->size.load() - n_take);
+    // If the data is unbalanced the list might be empty, which is actually nullptr
+    if (!rec_list) { return; }
+    auto dim = index.dim();
+    n_take   = std::min<uint32_t>(n_take, rec_list->size.load());
+    n_skip   = std::min<uint32_t>(n_skip, rec_list->size.load() - n_take);
 
     if (n_take == 0) { return; }