strings/ffs: avoid possible recursion

[lukileczo]

Description

Motivation and Context

Fixes: phoenix-rtos/phoenix-rtos-project#1005

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

How Has This Been Tested?

• Already covered by automatic testing.
• New test added: (add PR link here).
• Tested by hand on: (list targets here).

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing linter checks and tests passed.
• My changes generate no new compilation warnings for any of the targets.

Special treatment

• This PR needs additional PRs to work (list the PRs, preferably in merge-order).
• I will merge this PR by myself when appropriate.

[github-actions]

Unit Test Results

6 410 tests  ±0   5 780 ✅ ±0   33m 10s ⏱️ -1s
  354 suites ±0     630 💤 ±0 
    1 files   ±0       0 ❌ ±0 

Results for commit fdd0f5c. ± Comparison against base commit 4346c83.