XRefStm logic for trailers fixing

[BezrukovM]

closes https://github.com/veraPDF/veraPDF-parser/issues/354

[codecov]

Codecov Report

Merging #359 into integration will increase coverage by <.01%.
The diff coverage is 33.33%.

@@                Coverage Diff                @@
##             integration     #359      +/-   ##
=================================================
+ Coverage           25.2%   25.21%   +<.01%     
- Complexity           864      865       +1     
=================================================
  Files                233      233              
  Lines              12774    12782       +8     
  Branches            1806     1807       +1     
=================================================
+ Hits                3220     3223       +3     
- Misses              9181     9185       +4     
- Partials             373      374       +1

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/org/verapdf/cos/COSTrailer.java	15.62% <20%> (+0.81%)	3 <1> (+1)	⬆️
src/main/java/org/verapdf/parser/PDFParser.java	47.45% <42.85%> (+0.19%)	23 <0> (ø)	⬇️

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