Fix for invalid ClassDefinitionTable offset handling

[JimBobSquarePants]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following matches the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules 👮.
• I have provided test coverage for my change (where applicable)

Description

This fixes an issue discovered in the Stimulsoft forums linked from #354. The font contains an invalid offset to the ClassDefinitionTable. I've tested rendering the font and output is as expected when we return null.

[codecov]

Codecov Report

Merging #360 (85dd84f) into main (429d527) will increase coverage by 0%.
Report is 1 commits behind head on main.
The diff coverage is 100%.

@@          Coverage Diff          @@
##            main    #360   +/-   ##
=====================================
  Coverage     86%     86%           
=====================================
  Files        238     238           
  Lines      15171   15182   +11     
  Branches    2100    2100           
=====================================
+ Hits       13073   13085   +12     
  Misses      1613    1613           
+ Partials     485     484    -1     

Flag	Coverage Δ
unittests	86% <100%> (+<1%)	⬆️

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Files Changed	Coverage Δ
.../Tables/AdvancedTypographic/AttachmentListTable.cs	100% <100%> (ø)
...Tables/AdvancedTypographic/ClassDefinitionTable.cs	95% <100%> (+1%)	⬆️
...Tables/AdvancedTypographic/GlyphDefinitionTable.cs	84% <100%> (+1%)	⬆️
...s/Tables/AdvancedTypographic/MarkGlyphSetsTable.cs	100% <100%> (ø)

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[tocsoft]

looks good to me 👍