Fix string length in stripe dictionary building

[kaatish]

In PR #7676 the length of the current string being referred to while building stripe dictionaries was always set to 0 while incrementing the dictionary character count of a StripeDictionary. This led to corrupted strings when the dictionary encoding was used as noted in issue #7741. This has been fixed in this PR.

Fixes #7741

[vuule]

Note: no new tests added because the code path is disabled due to the issue fixed in #7737. The fix will be verified by CI on #7737 (once this is merged). Verified locally by combining the two fixes.

[codecov]

Codecov Report

Merging #7744 (9a67f5a) into branch-0.19 (7871e7a) will increase coverage by 0.67%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           branch-0.19    #7744      +/-   ##
===============================================
+ Coverage        81.86%   82.54%   +0.67%     
===============================================
  Files              101      101              
  Lines            16884    17461     +577     
===============================================
+ Hits             13822    14413     +591     
+ Misses            3062     3048      -14     

Impacted Files	Coverage Δ
python/cudf/cudf/core/column/lists.py	87.68% <0.00%> (-3.72%)	⬇️
python/cudf/cudf/core/column/decimal.py	92.95% <0.00%> (-1.92%)	⬇️
python/cudf/cudf/core/column/numerical.py	94.83% <0.00%> (-0.20%)	⬇️
python/cudf/cudf/core/column/column.py	87.61% <0.00%> (-0.15%)	⬇️
python/cudf/cudf/utils/utils.py	85.36% <0.00%> (-0.07%)	⬇️
python/cudf/cudf/io/feather.py	100.00% <0.00%> (ø)
python/cudf/cudf/utils/ioutils.py	78.71% <0.00%> (ø)
python/cudf/cudf/comm/serialize.py	0.00% <0.00%> (ø)
python/cudf/cudf/_fuzz_testing/io.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/column/struct.py	100.00% <0.00%> (ø)
... and 42 more

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[vuule]

@gpucibot merge