Fix undefined behavior in log

[mxgrey]

This PR fixes a case of undefined behavior that can happen in the task logging system.

Previously, when a log was read twice in a row without any new entries being written, the flawed logic in the reader would increment an iterator past the end of its container, which leads to undefined behavior. This PR fixes that logic and prevents the iterators from going past the end.

This branch builds off of the branch for #61 so we should merge that PR first.

[codecov]

Codecov Report

Merging #62 (9ad33ed) into main (a5e294a) will increase coverage by 0.27%.
The diff coverage is 44.92%.

@@            Coverage Diff             @@
##             main      #62      +/-   ##
==========================================
+ Coverage   36.21%   36.48%   +0.27%     
==========================================
  Files          65       65              
  Lines        2491     2557      +66     
  Branches     1366     1416      +50     
==========================================
+ Hits          902      933      +31     
+ Misses        552      550       -2     
- Partials     1037     1074      +37     

Flag	Coverage Δ
tests	36.48% <44.92%> (+0.27%)	⬆️

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Impacted Files	Coverage Δ
rmf_task/test/unit/test_Log.cpp	40.47% <44.26%> (+10.04%)	⬆️
rmf_task/src/rmf_task/Log.cpp	60.39% <50.00%> (-0.22%)	⬇️
rmf_task/include/rmf_task/Log.hpp	66.66% <0.00%> (+33.33%)	⬆️

[aaronchongth]

Looks great! Thanks for adding the test.

I don't believe it should be a concern, but just a random note, testing locally I see that it spins up to about 1GB of RAM during those 5 second tests.

[mxgrey]

@aaronchongth 5s and 1GB is admittedly pretty extreme. I've dialed it down to 1s, but unfortunately that made your review stale.