Divide by zero in Delta-Eddington routine

[kshedstrom]

I'm using one of the new initialization options for ice thickness such that I have roughly a meter of ice north of a specified latitude. The code fails in debug mode, runs otherwise:

Program received signal SIGFPE: Floating-point exception - erroneous arithmetic operation.
Backtrace for this error:
#0 0x7f2c3489756f in ???
#1 0x6eb30a in __ice_shortwave_dedd_MOD_compute_dedd0
	at //import/c1/AKWATERS/kate/ESMG/ESMG-configs/src/SIS2/src/ice_shortwave_dEdd.F90:1250

The ice area is all in category 1 while the ice mass is in category 3, leading to dz=0 for category 1.