zero out ponds when there level fraction is < puny

Author: davidclemenssewall

columnphysics/icepack_therm_vertical.F90

@@ -2749,7 +2749,11 @@ subroutine icepack_step_therm1(dt,                        &
 
             ! Translate changes in apond into apnd tracer
             if (tr_pond_lvl) then
-               apnd(n) = apond(n) / alvl(n)
+               if (alvl(n) > puny) then
+                  apnd(n) = max(apond(n) / alvl(n), c1)
+               else
+                  apnd(n) = c0
+               endif
             else
                apnd(n) = apond(n)
             endif