Refactor read_chunk to eliminate possible compiler warning

[alugowski]

Previous code could cause a compiler warning with overly-pedantic settings: 'value' may be used uninitialized

Previous behavior was correct, only initialized values would be read. New version also has simpler logic.

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (b8e3403) 95.84% compared to head (96d99ea) 95.77%.

Files	Patch %	Lines
include/fast_matrix_market/read_body.hpp	92.68%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #65      +/-   ##
==========================================
- Coverage   95.84%   95.77%   -0.07%     
==========================================
  Files          22       22              
  Lines        1563     1561       -2     
  Branches      176      173       -3     
==========================================
- Hits         1498     1495       -3     
- Misses         65       66       +1     

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