Fix overflow check in cudf::merge (#14345)

#14250 added a check to ensure `cudf::merge` throws when the total number of merged rows exceed `cudf::size_type` limit, however @bdice pointed out that the check was not correct because the accumulation was still occurring in `cudf::size_type`. This PR computes the accumulation in `std::size_t`.

Authors:
  - Divye Gala (https://github.com/divyegala)

Approvers:
  - Bradley Dice (https://github.com/bdice)
  - MithunR (https://github.com/mythrocks)
  - Nghia Truong (https://github.com/ttnghia)

URL: #14345

cpp/src/merge/merge.cu

@@ -638,13 +638,14 @@ table_ptr_type merge(std::vector<table_view> const& tables_to_merge,
 
   CUDF_EXPECTS(key_cols.size() == column_order.size(),
                "Mismatched size between key_cols and column_order");
-  CUDF_EXPECTS(std::accumulate(tables_to_merge.cbegin(),
-                               tables_to_merge.cend(),
-                               cudf::size_type{0},
-                               [](auto const& running_sum, auto const& tbl) {
-                                 return running_sum + tbl.num_rows();
-                               }) <= std::numeric_limits<cudf::size_type>::max(),
-               "Total number of merged rows exceeds row limit");
+  CUDF_EXPECTS(
+    std::accumulate(tables_to_merge.cbegin(),
+                    tables_to_merge.cend(),
+                    std::size_t{0},
+                    [](auto const& running_sum, auto const& tbl) {
+                      return running_sum + static_cast<std::size_t>(tbl.num_rows());
+                    }) <= static_cast<std::size_t>(std::numeric_limits<cudf::size_type>::max()),
+    "Total number of merged rows exceeds row limit");
 
   // This utility will ensure all corresponding dictionary columns have matching keys.
   // It will return any new dictionary columns created as well as updated table_views.