fix: Fix row_index of batched reader

[ritchie46]

fixes #19460

[codecov]

Codecov Report

Attention: Patch coverage is 87.87879% with 4 lines in your changes missing coverage. Please review.

Project coverage is 79.92%. Comparing base (ce001f1) to head (6621732).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-io/src/utils/other.rs	87.87%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #19465      +/-   ##
==========================================
- Coverage   79.95%   79.92%   -0.04%     
==========================================
  Files        1534     1534              
  Lines      211053   211123      +70     
  Branches     2444     2444              
==========================================
- Hits       168751   168743       -8     
- Misses      41747    41825      +78     
  Partials      555      555              

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