Floating point exception in PHS's plc function

### Brief summary of bug

When running in DEBUG mode, PHS's plc function generates a floating point exception in some circumstances. I'm not sure what happens in non-DEBUG mode.

### General bug information

**CTSM version you are using:** release-clm5.0.26

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

**Configurations affected:** Potentially all configurations with PHS (Clm50 default)? Observed in compset IHistClm50Sp with a start year of 1969 (I'm not sure if we'd observe this if we started from 1850).

### Details of bug

I set up a case as follows:

```
./create_newcase --case oob_0801b --res f19_g17 --compset IHistClm50Sp

./xmlchange RUN_STARTDATE=1969-01-01
./xmlchange STOP_OPTION=nyears,STOP_N=2
./xmlchange JOB_WALLCLOCK_TIME=1:00:00
./xmlchange DEBUG=TRUE
```

This ran for 5 months, but died on June 1 (model date 19690601) with error, `638:MPT ERROR: Rank 638(g:638) received signal SIGFPE(8).` and the following stack trace:

```
638:MPT: #7 0x0000000001d50eb3 in photosynthesismod::plc (x=3077.1112811201278,
638:MPT: p=49709, c=15947, level=4, plc_method=0)
638:MPT: at /glade/work/sacks/ctsm_code/ctsm/src/biogeophys/PhotosynthesisMod.F90:4888
638:MPT: #8 0x0000000001d3fabb in photosynthesismod::spacf (p=49709, c=15947, x=...,
638:MPT: f=..., qflx_sun=1.7423788723701508e-07, qflx_sha=3.3277860202329161e-09,
638:MPT: atm2lnd_inst=..., canopystate_inst=..., waterstate_inst=...,
638:MPT: soilstate_inst=..., temperature_inst=..., waterflux_inst=...)
638:MPT: at /glade/work/sacks/ctsm_code/ctsm/src/biogeophys/PhotosynthesisMod.F90:4653
638:MPT: #9 0x0000000001d2d125 in photosynthesismod::calcstress (p=49709, c=15947,
638:MPT: x=..., bsun=1, bsha=1, gb_mol=989500.02165873151,
638:MPT: gs_mol_sun=155650.16359247209, gs_mol_sha=155134.75791357172,
638:MPT: qsatl=0.0044985860266598997, qaf=0.0035459593784752008, atm2lnd_inst=...,
638:MPT: canopystate_inst=..., waterstate_inst=..., soilstate_inst=...,
638:MPT: temperature_inst=..., waterflux_inst=...)
638:MPT: at /glade/work/sacks/ctsm_code/ctsm/src/biogeophys/PhotosynthesisMod.F90:4293
638:MPT: #10 0x0000000001d0c6c9 in photosynthesismod::ci_func_phs (x=...,
638:MPT: cisun=29.108501370856779, cisha=29.10893561709543,
638:MPT: fvalsun=-6.8697493455910248, fvalsha=-6.8701739416686838, p=49709, iv=1,
638:MPT: c=15947, bsun=1, bsha=1, bflag=4294967295, gb_mol=989500.02165873151,
638:MPT: gs0sun=155650.16359247209, gs0sha=155134.75791357172,
638:MPT: gs_mol_sun=155650.16359247209, gs_mol_sha=155134.75791357172,
638:MPT: jesun=17.169554658508506, jesha=17.061857082605716,
638:MPT: cair=31.773491626981734, oair=20547.941476562406,
638:MPT: lmr_z_sun=0.028769883436967889, lmr_z_sha=0.028769883436967889,
638:MPT: par_z_sun=238.05876325366955, par_z_sha=180.08649238896118,
638:MPT: rh_can=0.14863687114448806, qsatl=0.0044985860266598997,
638:MPT: qaf=0.0035459593784752008, atm2lnd_inst=..., photosyns_inst=...,
638:MPT: canopystate_inst=..., waterstate_inst=..., soilstate_inst=...,
638:MPT: temperature_inst=..., waterflux_inst=...)
638:MPT: at /glade/work/sacks/ctsm_code/ctsm/src/biogeophys/PhotosynthesisMod.F90:4036
638:MPT: #11 0x0000000001cfec86 in photosynthesismod::hybrid_phs (
638:MPT: x0sun=29.108501370856779, x0sha=29.10893561709543, p=49709, iv=1, c=15947,
638:MPT: gb_mol=989500.02165873151, bsun=1, bsha=1, jesun=17.169554658508506,
638:MPT: jesha=17.061857082605716, cair=31.773491626981734,
638:MPT: oair=20547.941476562406, lmr_z_sun=0.028769883436967889,
638:MPT: lmr_z_sha=0.028769883436967889, par_z_sun=238.05876325366955,
638:MPT: par_z_sha=180.08649238896118, rh_can=0.14863687114448806,
638:MPT: gs_mol_sun=155650.16359247209, gs_mol_sha=155134.75791357172,
638:MPT: qsatl=0.0044985860266598997, qaf=0.0035459593784752008, iter1=2, iter2=0,
638:MPT: atm2lnd_inst=..., photosyns_inst=..., canopystate_inst=...,
638:MPT: waterstate_inst=..., soilstate_inst=..., temperature_inst=...,
638:MPT: waterflux_inst=...)
638:MPT: at /glade/work/sacks/ctsm_code/ctsm/src/biogeophys/PhotosynthesisMod.F90:3661
638:MPT: #12 0x0000000001cce59c in photosynthesismod::photosynthesishydraulicstress (
638:MPT: bounds=..., fn=17, filterp=..., esat_tv=..., eair=..., oair=..., cair=...,
638:MPT: rb=..., bsun=..., bsha=..., btran=..., dayl_factor=..., leafn=...,
638:MPT: qsatl=..., qaf=..., atm2lnd_inst=..., temperature_inst=...,
638:MPT: soilstate_inst=..., waterstate_inst=..., surfalb_inst=...,
638:MPT: solarabs_inst=..., canopystate_inst=..., ozone_inst=0x2b27ca63fd80,
638:MPT: photosyns_inst=..., waterflux_inst=..., froot_carbon=..., croot_carbon=...)
638:MPT: at /glade/work/sacks/ctsm_code/ctsm/src/biogeophys/PhotosynthesisMod.F90:3326
638:MPT: #13 0x00000000011c04eb in canopyfluxesmod::canopyfluxes (bounds=...,
638:MPT: num_exposedvegp=17, filter_exposedvegp=..., clm_fates=..., nc=1,
638:MPT: atm2lnd_inst=..., canopystate_inst=..., energyflux_inst=...,
638:MPT: frictionvel_inst=..., soilstate_inst=..., solarabs_inst=...,
638:MPT: surfalb_inst=..., temperature_inst=..., waterflux_inst=...,
638:MPT: waterstate_inst=..., ch4_inst=..., ozone_inst=0x2b27ca63fd80,
638:MPT: photosyns_inst=..., humanindex_inst=...,
638:MPT: soil_water_retention_curve=0x2b27ca462080, downreg_patch=...,
638:MPT: leafn_patch=..., froot_carbon=..., croot_carbon=...)
638:MPT: at /glade/work/sacks/ctsm_code/ctsm/src/biogeophys/CanopyFluxesMod.F90:852
638:MPT: #14 0x00000000008848ed in clm_driver::clm_drv (doalb=4294967295,
638:MPT: nextsw_cday=153, declinp1=0.38482293161813563, declin=0.38482293161813563,
638:MPT: rstwr=.FALSE., nlend=.FALSE., rdate=..., rof_prognostic=4294967295,
638:MPT: .tmp.RDATE.len_V$202d6=32)
638:MPT: at /glade/work/sacks/ctsm_code/ctsm/src/main/clm_driver.F90:543
638:MPT: #15 0x00000000008470fe in lnd_comp_mct::lnd_run_mct (eclock=..., cdata_l=...,
638:MPT: x2l_l=..., l2x_l=...)
638:MPT: at /glade/work/sacks/ctsm_code/ctsm/src/cpl/lnd_comp_mct.F90:456
638:MPT: #16 0x0000000000461bc1 in component_mod::component_run (eclock=..., comp=...,
638:MPT: infodata=..., seq_flds_x2c_fluxes=..., seq_flds_c2x_fluxes=...,
638:MPT: comp_prognostic=4294967295, comp_num=2, timer_barrier=...,
638:MPT: timer_comp_run=..., run_barriers=.FALSE., ymd=19690602, tod=0,
638:MPT: comp_layout=..., .tmp.SEQ_FLDS_X2C_FLUXES.len_V$3526=4096,
638:MPT: .tmp.SEQ_FLDS_C2X_FLUXES.len_V$3529=4096,
638:MPT: .tmp.TIMER_BARRIER.len_V$352e=19, .tmp.TIMER_COMP_RUN.len_V$3531=11,
638:MPT: .tmp.COMP_LAYOUT.len_V$3537=32)
638:MPT: at /glade/work/sacks/ctsm_code/ctsm/cime/src/drivers/mct/main/component_mod.F90:728
638:MPT: #17 0x00000000004308d8 in cime_comp_mod::cime_run ()
638:MPT: at /glade/work/sacks/ctsm_code/ctsm/cime/src/drivers/mct/main/cime_comp_mod.F90:2712
638:MPT: #18 0x0000000000449570 in cime_driver ()
638:MPT: at /glade/work/sacks/ctsm_code/ctsm/cime/src/drivers/mct/main/cime_driver.F90:125
```

It looks like the issue may be that x is positive in the call to plc, so (x/psi50) is negative, and we get a floating point exception when trying to raise this negative value to a real power. Here x appears to be the root water potential.

This bug was originally found by @Ivanderkelen ; I have reproduced it.