Fix race condition in OnDiskInvertedlists. (#464)

OnDiskInvertedLists.cpp

@@ -164,6 +164,7 @@ struct OnDiskInvertedLists::OngoingPrefetch {
     static void* prefetch_list (void * arg) {
         Thread *th = static_cast<Thread*>(arg);
 
+        th->od->locks->lock_1(th->list_no);
         size_t n = th->od->list_size(th->list_no);
         const Index::idx_t *idx = th->od->get_ids(th->list_no);
         const uint8_t *codes = th->od->get_codes(th->list_no);
@@ -177,6 +178,7 @@ struct OnDiskInvertedLists::OngoingPrefetch {
         for (size_t i = 0; i < n8;i++) {
             cs += codes8[i];
         }
+        th->od->locks->unlock_1(th->list_no);
         global_cs += cs & 1;
         return nullptr;
     }