compress_entry: fix assertion failure

[ThomasWaldmann]

if the size is not known yet in the chunks index, we must update the value there.

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 81.74%. Comparing base (8a13cf2) to head (0545b6a).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
src/borg/cache.py	33.33%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8516      +/-   ##
==========================================
- Coverage   81.76%   81.74%   -0.03%     
==========================================
  Files          73       73              
  Lines       13140    13142       +2     
  Branches     1926     1927       +1     
==========================================
- Hits        10744    10743       -1     
- Misses       1740     1741       +1     
- Partials      656      658       +2     

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