Fixed bug in reading DecodeParems seen with CCITT images in gov.uscou…

[peterwilliams97]

To test :

cd $GOPATH/src/github.com/unidoc/unidoc-examples/pdf/image
go run pdf_list_images.go gov.uscourts.ctd.18812.88.0.pdf

gov.uscourts.ctd.18812.88.0.pdf is in https://github.com/peterwilliams97/unidoc-testdata

[codecov]

Codecov Report

Merging #356 into v3 will decrease coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##               v3    #356      +/-   ##
=========================================
- Coverage   55.21%   55.2%   -0.01%     
=========================================
  Files         152     152              
  Lines       27299   27302       +3     
=========================================
  Hits        15073   15073              
- Misses      10372   10375       +3     
  Partials     1854    1854

Impacted Files	Coverage Δ
pdf/core/encoding.go	44.28% <0%> (-0.11%)	⬇️

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