Add missing \expandafter for gh1544

[u-fischer]

READ ME FIRST: Please understand that in most cases we will not be able to merge a pull request because there are a lot of internal activities needed when updating the LaTeX2e sources. If you have a code suggestion please discuss it with the team first.

Internal housekeeping

Status of pull request

• Ready to merge

Checklist of required changes before merge will be approved

• [] Test file(s) added (don't think another one is needed)
• [x] Version and date string updated in changed source files
• [x] Relevant \changes entries in source included
• [x] Relevant changes.txt updated
• [ ] Rollback provided (if necessary)?
• [ ] ltnewsX.tex (and/or latexchanges.tex) updated