Fix a bug in forecast_predet.sh

[XianwuXue-NOAA]

Expected behavior

When restart_interval_gfs=0, then FHMAX_GFS % restart_interval_gfs will cause error

Current behavior

When restart_interval_gfs=0, then FHMAX_GFS % restart_interval_gfs will cause error

Machines affected

all machines when restart_interval_gfs=0

To Reproduce

Run coupled model