Do not crash if zim file has no Counter metadata.

[mgautierfr]

Fix #447

[codecov]

Codecov Report

Merging #449 (4749656) into master (e0bcafd) will decrease coverage by 0.00%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master     #449      +/-   ##
==========================================
- Coverage   59.78%   59.77%   -0.01%     
==========================================
  Files          51       51              
  Lines        3414     3416       +2     
  Branches     1732     1721      -11     
==========================================
+ Hits         2041     2042       +1     
- Misses       1370     1371       +1     
  Partials        3        3              

Impacted Files	Coverage Δ
src/reader.cpp	48.37% <75.00%> (+0.01%)	⬆️

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