Fix libcudf compile warnings on debug 11.4 build

[davidwendt]

This fixes some compile warnings recently added in #9299 and #9278. These warnings are turned into errors on a libcudf Debug build.

[codecov]

Codecov Report

Merging #9360 (470d328) into branch-21.12 (ab4bfaa) will decrease coverage by 0.03%.
The diff coverage is 1.82%.

❗ Current head 470d328 differs from pull request most recent head 53fb18d. Consider uploading reports for the commit 53fb18d to get more accurate results

@@               Coverage Diff                @@
##           branch-21.12    #9360      +/-   ##
================================================
- Coverage         10.79%   10.75%   -0.04%     
================================================
  Files               116      116              
  Lines             18869    19076     +207     
================================================
+ Hits               2036     2051      +15     
- Misses            16833    17025     +192     

Impacted Files	Coverage Δ
python/cudf/cudf/_lib/__init__.py	0.00% <ø> (ø)
python/cudf/cudf/core/_base_index.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/column/categorical.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/column/column.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/column/datetime.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/column/lists.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/column/numerical.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/column/string.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/column/timedelta.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/dataframe.py	0.00% <0.00%> (ø)
... and 26 more

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[vyasr]

LGTM

[vyasr]

rerun tests

[davidwendt]

@gpucibot merge