Avoid divide by zero

This should only affect tests with the oldhyd test mod. However, it is
needed in order for these tests to pass:

ERP_D_Ld5.f10_f10_musgs.I2000Clm50BgcCrop.yellowstone_intel.clm-allActive
ERP_D_P15x2_Ld3.f10_f10_musgs.I2000Clm50BgcCrop.yellowstone_intel.clm-cropColdStart
ERP_P180x2_D_Ld5.f19_g17.I2000Clm50BgcDvCrop.yellowstone_pgi.clm-crop
ERP_D_Ld5.f10_f10_musgs.I2000Clm50BgcCrop.cheyenne_intel.clm-allActive
ERP_D_P15x2_Ld3.f10_f10_musgs.I2000Clm50BgcCrop.cheyenne_intel.clm-cropColdStart

src/biogeophys/SoilHydrologyMod.F90

@@ -165,7 +165,12 @@ subroutine SetSoilWaterFractions(bounds, num_hydrologyc, filter_hydrologyc, &
 
           ! fracice is only used in code with origflag == 1. For this calculation, we use
           ! the version of icefrac that was used in this original hydrology code.
-          icefrac_orig = min(1._r8,h2osoi_ice(c,j)/(h2osoi_ice(c,j)+h2osoi_liq(c,j)))
+          if (h2osoi_ice(c,j) == 0._r8) then
+             ! Avoid possible divide by zero (in case h2osoi_liq(c,j) is also 0)
+             icefrac_orig = 0._r8
+          else
+             icefrac_orig = min(1._r8,h2osoi_ice(c,j)/(h2osoi_ice(c,j)+h2osoi_liq(c,j)))
+          end if
           fracice(c,j) = max(0._r8,exp(-3._r8*(1._r8-icefrac_orig))- exp(-3._r8))/(1.0_r8-exp(-3._r8))
        end do
     end do