Fix a memory leak in getUseridByExternalInfo (v2)

[ifakhrutdinov]

Proposed changes

The PR adds a missing FREE_STRUCT31 macro invocation to the branch taken when a bad function code is passed to getUseridByExternalInfo.

This PR addresses Issue: #467

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

PR Checklist

Please delete options that are not relevant.

• If the changes in this PR are meant for the next release / mainline, this PR targets the "staging" branch.
• My code follows the style guidelines of this project (see: Contributing guideline)
• New and existing unit tests pass locally with my changes
• Relevant update to CHANGELOG.md
• My changes generate no new warnings

Testing

Regression testing of the getUseridByExternalInfo function in c/rsusermap.c is needed.