z = 0 for invalid number of processes

e.g. using 24 processes for a 128x128x128 grid would need 21 processes
with z=6 and 1 process with z=2

src/grid/discretization_grid.f90

@@ -84,6 +84,8 @@ subroutine discretization_grid_init(restart)
                                    PETSC_COMM_WORLD, &
                                    z, &                                                             ! domain grid size along z
                                    z_offset)                                                        ! domain grid offset along z
+  if(z==0_C_INTPTR_T) call IO_error(894, ext_msg='invalid number of threads')
+
   grid3       = int(z)
   grid3Offset = int(z_offset)
   size3       = geomSize(3)*real(grid3,pReal)      /real(grid(3),pReal)