Don't check size of the last POS file

[poszu]

Previously it wrongly reported a corrupted POS data if the last file was smaller than max file size. It's fine, the last file can be smaller.

[codecov]

Codecov Report

Merging #31 (8504f76) into main (99f1358) will increase coverage by 0.0%.
The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main     #31   +/-   ##
=====================================
  Coverage   96.7%   96.8%           
=====================================
  Files         12      12           
  Lines       1031    1033    +2     
=====================================
+ Hits         998    1000    +2     
  Misses        33      33           

Impacted Files	Coverage Δ
src/reader.rs	99.2% <100.0%> (+<0.1%)	⬆️

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