fix split cells

[willmcgugan]

Fixed Segment._split_cells issue discovered in Textualize/textual#4996

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.26%. Comparing base (9c74f03) to head (23c64f1).
Report is 29 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3506      +/-   ##
==========================================
+ Coverage   98.25%   98.26%   +0.01%     
==========================================
  Files          74       74              
  Lines        8072     8090      +18     
==========================================
+ Hits         7931     7950      +19     
+ Misses        141      140       -1     

Flag	Coverage Δ
unittests	98.26% <100.00%> (+0.01%)	⬆️

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