[slang-tidy] Fix segmentation fault during mapping checker name on the group

[likeamahoney]

There is a segfault when existing checker is mapped to else's group

For example (segfault - no-old-always-syntax is from style group but not synthesis):

./build/gcc-11-release/bin/slang-tidy --config-file="Checks:-*,synthesis-no-old-always-syntax" test.sv

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (de03987) 93.76% compared to head (fd26280) 93.76%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #883   +/-   ##
=======================================
  Coverage   93.76%   93.76%           
=======================================
  Files         189      189           
  Lines       46866    46866           
=======================================
  Hits        43942    43942           
  Misses       2924     2924           

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