print_MARBL_log sometimes hangs on error instead of cleanly exiting

### Description of the issue:
@kristenkrumhardt pointed this out -- under some situations, she was seeing

```
ERROR reported from MARBL library
```

in `cesm.log` but then `MPI_Abort()` was never triggered. It looks like something funny is happening in `exit_POP()`?

### Version:
- CESM: 2.2.0
- POP2: `cesm_pop_2_1_20200730`

### Machine/Environment Description:

cheyenne

### Any xml/namelist changes or SourceMods:

I was able to recreate this error with the following steps:

1. create a `G1850ECOIAF` compset with the `T62_g17` resolution
2. copy `/glade/work/kristenk/cesm_work/cesm2.2_4P4Z_tuningcases/g.e22a06.G1850ECOIAF.T62_g17.4p4z.001/user_nl_marbl3.old` into the case directory as `user_nl_marbl` (to enable 4 autotrophs / 4 zooplankton)
3. add

```
init_ecosys_init_file='/glade/u/home/kristenk/adding_zooplankton/IC_files/ecosys_jan_IC_4p4z_200906.nc'
```

to `user_nl_pop`.

See the case in `/glade/work/mlevy/codes/CESM/cesm2.2.0/cases/g.e22.G1850ECOIAF.T62_g17.hang_not_ab
ort` (rundir: `/glade/scratch/mlevy/g.e22.G1850ECOIAF.T62_g17.hang_not_abort/run`)