Fix a missing line regarding vel_underflow

[herrwang0]

This patches issue https://github.com/NOAA-GFDL/MOM6/issues/1336

[codecov]

Codecov Report

Merging #1340 (8d0a80f) into dev/gfdl (f06669e) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##           dev/gfdl    #1340   +/-   ##
=========================================
  Coverage     45.75%   45.75%           
=========================================
  Files           234      234           
  Lines         72555    72556    +1     
=========================================
+ Hits          33194    33195    +1     
  Misses        39361    39361           

Impacted Files	Coverage Δ
src/core/MOM_barotropic.F90	68.03% <100.00%> (+0.01%)	⬆️

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[marshallward]

https://gitlab.gfdl.noaa.gov/ogrp/MOM6/-/pipelines/12152 ✔️

[marshallward]

A straightforward fix, restores symmetry of the underflow operation.