Avoid an infinite loop on cyclic field Parent chains

[calixteman]

When an AcroForm has no Fields entry, #fixFields rebuilds it by walking each widget's Parent chain up to its root field. A chain looping back on itself made that walk spin forever and hung the worker.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.02%. Comparing base (b4ba666) to head (a7a99fd).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #21699   +/-   ##
=======================================
  Coverage   90.01%   90.02%           
=======================================
  Files         264      264           
  Lines       66893    66904   +11     
=======================================
+ Hits        60214    60227   +13     
+ Misses       6679     6677    -2     

Flag	Coverage Δ
browsertest	66.52% <0.00%> (-0.02%)	⬇️
fonttest	9.03% <ø> (ø)
integrationtest	69.33% <0.00%> (-0.01%)	⬇️
unittest	57.88% <100.00%> (+<0.01%)	⬆️
unittestcli	56.61% <100.00%> (+0.01%)	⬆️

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[timvandermeij]

Nice find!