Bugfix/segfault when getting surrounding interval of empty cache (backport #116)

[mergify]

This PR fixes a segfault that occurred to me when I tried to get a surrounding interval from a message_filter::Cache that was empty. See comment below for where/how.

The PR includes

• a test that should reproduce the segfault when executed without the fix.
• the fix of the segfault

Is it possible to get the fix also to other branches, specifically humble (but iron would probably also make sense)? That's the version I am using at the moment. I think the fix commit should be cherry-pickable. But the test commit probably needs some minor adaptation due to namespace changes.

Please let me know if I need to follow some specific procedure or provide more information for contributing (e.g. create issue first, or similar).

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This is an automatic backport of pull request #116 done by Mergify.

[mergify]

Cherry-pick of e60450d has failed:

On branch mergify/bp/jazzy/pr-116
Your branch is up to date with 'origin/jazzy'.

You are currently cherry-picking commit e60450d.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   test/msg_cache_unittest.cpp

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   include/message_filters/cache.hpp

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally