Fix mis-match condition in replacement question array

[marcellamaki]

Summary

Fixes #12335

References

Reviewer guidance

Are the proper number of replacements reflected in the replacement selection once you have added more than the current number of questions to swap?

replace-questions.mp4

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Testing checklist

• Contributor has fully tested the PR manually
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• Critical and brittle code paths are covered by unit tests

PR process

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Reviewer checklist

• Automated test coverage is satisfactory
• PR is fully functional
• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md

[github-actions]

Build Artifacts

Asset type	Download link
PEX file	kolibri-.pex
Windows Installer (EXE)	kolibri-0.17.0a0.dev0+git.145.g5de86661-windows-setup-unsigned.exe
Debian Package	kolibri_0.17.0a0.dev0+git.145.g5de86661-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.17.0a0.dev0+git.145.g5de86661-0.4.2.dmg
Android Package (APK)	kolibri-0.17.0a0.dev0+git.145.g5de86661-0.1.3-debug.apk
TAR file	kolibri-0.17.0a0.dev0+git.145.g5de86661.tar.gz
WHL file	kolibri-0.17.0a0.dev0+git.145.g5de86661-py2.py3-none-any.whl

[rtibbles]

Probably appears to fix the change but might introduce another, very subtle, bug.

[rtibbles]

Code changes make sense, and the behavioural fix is confirmed.