fix: prevent matching STOP in a valid non-STOP sequence

[barabo]

When AUG and AUG (also UUA and AUG) are combined, they contain a substring that potentially matches a STOP codon. A naive implementation may falsely match the non-STOP sequence as a STOP codon.

This simple test covers a gap in the current suite.

  AUGAUG
   ^^^ - not a STOP codon

  UUAAUG
   ^^^ - not a STOP codon

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[barabo]

Originally created (in error) in the jq track: exercism/jq#242

[barabo]

Thanks for the quick review!