predefine area_null before passing to zero_areas
predefine area_null before passing to zero_areas

I don't know why, but my previous changes were causing a regression with
the sensor case in which the number of elements was off by 2.  No idea
why.  I will dig in more, but just want to get it working first.

barbagroup/pygbe,barbagroup/pygbe,barbagroup/pygbe