fix: avoid stack overflow in traverse

[barendgehrels]

See #1411 (last commit) where it was discovered and fixed.
This commit is meant to be merged to 1.89 beta

[tinko92]

Looks good to me (the hide whitespace is very helpful) and adds no failures or warnings to the test suite run afaics.

[barendgehrels]

Per mail today

Calendar is here: https://www.boost.org/calendar

Current status of the master branch:
Master is open for bug fixes and documentation updates. Other changes by permission of a release manager.
The release managers

@vissarion @tinko92 I will merge this to our master branch tonight or tomorrow. It is open until August 6

[barendgehrels]

@vissarion @tinko92 this is cherry-picked into the master branch:
617d75e