Fix overflow error for NNI distance between large trees

[ms609]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 83.78378% with 12 lines in your changes missing coverage. Please review.

Project coverage is 98.82%. Comparing base (47353a7) to head (15e8f05).
Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
src/tree_distances.cpp	75.00%	7 Missing ⚠️
src/spr.cpp	69.23%	4 Missing ⚠️
src/nni_distance.cpp	96.96%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #129      +/-   ##
==========================================
- Coverage   99.73%   98.82%   -0.92%     
==========================================
  Files          37       37              
  Lines        2605     2628      +23     
==========================================
- Hits         2598     2597       -1     
- Misses          7       31      +24     

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