Set default values to crashing

Because variables in `grid_nml` should be all defined in the input file.

Overlap/Tests/test_overlap.f90

@@ -32,7 +32,7 @@ program test_overlap
   ! longitude and latitude of the corner of the whole grid, in degrees
   ! and in rad
 
-  real:: step_deg(2) = [0.25, 0.25], step(2)
+  real:: step_deg(2) = [huge(0.), huge(0.)], step(2)
   ! longitude and latitude steps, in degrees and in rad
 
   logical periodic ! grid is periodic in longitude