Fix memcheck error in gtest SegmentedGatherTest/GatherSliced

[davidwendt]

Recent run of cuda-memcheck on gtests/COPYING_TEST uncovered an error in the segmented_gather_list_tests.cpp logic for GatherSliced. The memcheck reported a read error on the gather-map which was created with a temporary lists column that was freed before passing it to the cudf::lists::detail::segmented_gather function. Fixing the testcase fixed the memcheck error.

[codecov]

Codecov Report

Merging #9442 (faea7a6) into branch-21.12 (ab4bfaa) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

❗ Current head faea7a6 differs from pull request most recent head 9fe27a7. Consider uploading reports for the commit 9fe27a7 to get more accurate results

@@               Coverage Diff                @@
##           branch-21.12    #9442      +/-   ##
================================================
- Coverage         10.79%   10.77%   -0.02%     
================================================
  Files               116      117       +1     
  Lines             18869    19046     +177     
================================================
+ Hits               2036     2052      +16     
- Misses            16833    16994     +161     

Impacted Files	Coverage Δ
python/cudf/cudf/__init__.py	0.00% <0.00%> (ø)
python/cudf/cudf/_lib/__init__.py	0.00% <ø> (ø)
python/cudf/cudf/_lib/strings/__init__.py	0.00% <0.00%> (ø)
python/cudf/cudf/io/csv.py	0.00% <0.00%> (ø)
python/cudf/cudf/io/orc.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/frame.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/index.py	0.00% <0.00%> (ø)
python/cudf/cudf/io/parquet.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/series.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/reshape.py	0.00% <0.00%> (ø)
... and 30 more

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[davidwendt]

@gpucibot merge