Fix bug in ts.pair_coalescence_counts when a window breakpoint falls within missing interval

[nspope]

Fixes #3175 via a faulty interval intersection in the C implementation, and adds a test that would fail on main

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.62%. Comparing base (4195624) to head (48b52c2).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3176   +/-   ##
=======================================
  Coverage   89.62%   89.62%           
=======================================
  Files          28       28           
  Lines       31969    31969           
  Branches     5873     5873           
=======================================
  Hits        28653    28653           
  Misses       1886     1886           
  Partials     1430     1430           

Flag	Coverage Δ
c-tests	86.66% <100.00%> (ø)
lwt-tests	80.38% <ø> (ø)
python-c-tests	88.18% <ø> (ø)
python-tests	98.85% <ø> (ø)

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Files with missing lines	Coverage Δ
c/tskit/trees.c	90.61% <100.00%> (ø)

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[jeromekelleher]

Nice catch, LGTM