iox-#743 Validate callbacks in c example

[FerdinandSpitzschnueffler]

Pre-Review Checklist for the PR Author

1. Code follows the coding style of CONTRIBUTING.md
2. Tests follow the best practice for testing
3. Changelog updated in the unreleased section including API breaking changes
4. Branch follows the naming format (iox-#123-this-is-a-branch)
5. Commits messages are according to this guideline
   • Commit messages have the issue ID (iox-#123 commit text)
   • Commit messages are signed (git commit -s)
   • Commit author matches Eclipse Contributor Agreement (and ECA is signed)
6. Update the PR title
   • Follow the same conventions as for commit messages
   • Link to the relevant issue
7. Relevant issues are linked
8. Add sensible notes for the reviewer
9. All checks have passed (except task-list-completed)
10. Assign PR to reviewer

Notes for Reviewer

Checklist for the PR Reviewer

• Commits are properly organized and messages are according to the guideline
• Code according to our coding style and naming conventions
• Unit tests have been written for new behavior
  • Each unit test case has a unique UUID
• Public API changes are documented via doxygen
• Copyright owner are updated in the changed files
• PR title describes the changes

Post-review Checklist for the PR Author

1. All open points are addressed and tracked via issues

References

• Partially closes Improve documentation and website for v2.0 #743

[codecov]

Codecov Report

Merging #1188 (c587867) into master (3f1db77) will increase coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1188      +/-   ##
==========================================
+ Coverage   79.06%   79.12%   +0.06%     
==========================================
  Files         370      370              
  Lines       14660    14660              
  Branches     2046     2046              
==========================================
+ Hits        11591    11600       +9     
+ Misses       2399     2393       -6     
+ Partials      670      667       -3     

Flag	Coverage Δ
unittests	78.32% <ø> (+0.03%)	⬆️
unittests_timing	13.92% <ø> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
iceoryx_hoofs/source/concurrent/loffli.cpp	88.57% <0.00%> (-2.86%)	⬇️
iceoryx_posh/source/runtime/posh_runtime_impl.cpp	66.91% <0.00%> (+1.48%)	⬆️
iceoryx_hoofs/source/posix_wrapper/timer.cpp	64.73% <0.00%> (+1.65%)	⬆️

[elfenpiff]

Looks good, when elBoberido's finding is fixed it is ready to merge.