Add validation for EXTH record length in MobiParser

[baseciq]

Fixes #113

Avoid reading 0-sized records

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 90.07%. Comparing base (e6adbf3) to head (4b9cef9).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/Formats/Mobi/Parser/MobiParser.php	33.33%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #114      +/-   ##
============================================
- Coverage     90.13%   90.07%   -0.07%     
- Complexity      837      843       +6     
============================================
  Files            32       32              
  Lines          2586     2589       +3     
============================================
+ Hits           2331     2332       +1     
- Misses          255      257       +2     

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[ewilan-riviere]

Thanks for this PR!