Hillslope test fails with ifx compiler

### Brief summary of bug

`ERP_D_P64x2_Ld10.f10_f10_mg37.I2000Clm60Bgc.derecho_intel.clm-Hillslope` is failing in RUN during my testing of #3787 (at hash 60a9c5d3d). I can't find a netCDF file in its `lnd_in` that would cause this.

### General bug information

**CTSM version you are using:** Branch in #3787 at hash 60a9c5d3d.

**Does this bug cause significantly incorrect results in the model's science?** No

**Configurations affected:** Probably just hillslope runs; unclear if it's just this test.

### Details of bug

"Floating invalid" with unclear traceback. See `/glade/derecho/scratch/samrabin/tests_0301-165703de/ERP_D_P64x2_Ld10.f10_f10_mg37.I2000Clm60Bgc.derecho_intel.clm-Hillslope.GC.0301-165703de_int/run/cesm.log.5244584.desched1.260301-171820`.