Safer casting from double to long in value.hpp (backport #1831)

[mergify]

This is an automatic backport of pull request #1831 done by Mergify.

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[codecov]

Codecov Report

Merging #1837 (4674fae) into main (47de70b) will increase coverage by 0.02%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##             main    #1837      +/-   ##
==========================================
+ Coverage   60.58%   60.60%   +0.02%     
==========================================
  Files          96       96              
  Lines       18864    18874      +10     
  Branches     9483     9487       +4     
==========================================
+ Hits        11429    11439      +10     
+ Misses       5146     5144       -2     
- Partials     2289     2291       +2     

Impacted Files	Coverage Δ
include/exiv2/value.hpp	82.51% <80.00%> (-0.15%)	⬇️
src/tiffvisitor_int.cpp	75.50% <0.00%> (+0.22%)	⬆️

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