clang-tidy: use dynamic cast

[neheb]

Found with cppcoreguidelines-pro-type-static-cast-downcast

Signed-off-by: Rosen Penev rosenp@gmail.com

[codecov]

Codecov Report

Merging #1670 (7ed2c46) into main (8dbc699) will decrease coverage by 0.01%.
The diff coverage is 12.50%.

@@            Coverage Diff             @@
##             main    #1670      +/-   ##
==========================================
- Coverage   66.89%   66.88%   -0.02%     
==========================================
  Files         151      151              
  Lines       20747    20743       -4     
==========================================
- Hits        13879    13874       -5     
- Misses       6868     6869       +1     

Impacted Files	Coverage Δ
samples/Jzon.cpp	33.44% <0.00%> (-0.63%)	⬇️
samples/exiv2json.cpp	75.60% <50.00%> (ø)

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