SymbolTable: Gracefully handle current CHECK-fail

[hzeller]

This situation seems to arise if we have possibly multiple parent references when fed a bunch of possibly unrelated files with naming overlap. Don't crash in this case.

[codecov-commenter]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (470e0b9) 92.85% compared to head (3681b01) 92.84%.
Report is 6 commits behind head on master.

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2029      +/-   ##
==========================================
- Coverage   92.85%   92.84%   -0.01%     
==========================================
  Files         357      357              
  Lines       26399    26396       -3     
==========================================
- Hits        24512    24507       -5     
- Misses       1887     1889       +2     

Files	Coverage Δ
common/text/concrete_syntax_tree.cc	100.00% <100.00%> (ø)
common/text/concrete_syntax_tree.h	96.82% <100.00%> (+0.46%)	⬆️
common/text/tree_builder_test_util.cc	100.00% <100.00%> (ø)
common/text/tree_utils.cc	97.27% <100.00%> (-0.02%)	⬇️
common/text/tree_utils.h	100.00% <100.00%> (ø)
verilog/CST/expression.cc	96.10% <100.00%> (-0.10%)	⬇️
verilog/CST/macro.cc	100.00% <100.00%> (ø)
verilog/CST/module.cc	100.00% <100.00%> (ø)
verilog/CST/port.cc	94.36% <100.00%> (ø)
verilog/CST/seq_block.cc	100.00% <100.00%> (ø)
... and 15 more

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.