lib_base: Fix bug in Time.Protocol with negative seconds

Merged as part of !2721 (merged)

Context

Extract the bug fix found in !2688 (merged)

Manually testing the MR

$ dune exec src/lib_base/test/test_time.exe

Without the fix, the added test throws.

Checklist

• N/A Document the interface of any function added or modified (see the coding guidelines)
• Provide automatic testing (see the testing guide).
• N/A For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, the Development Version section of CHANGES.md for everything else).
• Select suitable reviewers using the Reviewers field below.