fread: avoid UB in leap year cycle calculation

[aitap]

Use the fix suggested by @kevinushey. Since R's Date class (and thus IDate) uses year zero, the resulting calculations agree with the system tzcode, although the Date class generally doesn't like to parse negative years:

$ TZ=UTC LC_ALL=C date -d '0004-02-29 -8year 12:00:00'
Thu Feb 29 12:00:00 UTC -004
$ R -q -s -e 'data.table::fread("-4-02-29 12:00:00\n")'
         V1       V2
     <IDat>   <char>
1: -4-02-29 12:00:00

Fixes: #7704

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.04%. Comparing base (8364344) to head (ac14cd5).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7720      +/-   ##
==========================================
- Coverage   99.04%   99.04%   -0.01%     
==========================================
  Files          87       87              
  Lines       17037    17123      +86     
==========================================
+ Hits        16874    16959      +85     
- Misses        163      164       +1     

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