ice_grid: initialize 'tarea' to 0

In subroutine 'init_grid2', the array 'tarea' is initialized in a loop
on indices {ilo, ihi} and {jlo, jhi}, but is then used in subroutine
'makemask' in a loop on indices {1, nx_block} and {1, ny_block},
potentially causing an uninitialized value to be used (at line 1693).

Initialize the whole array to zero first, to avoid using uninitialized values.
The initialization to zero is actually commented out, following
10c446a (Dummy and unused variables. (#180), 2018-09-22) [1] , so
uncomment it.

[1] #180

cicecore/cicedynB/infrastructure/ice_grid.F90

@@ -384,7 +384,7 @@ subroutine init_grid2
       ! T-grid cell and U-grid cell quantities
       !-----------------------------------------------------------------
 
-!     tarea(:,:,:) = c0
+      tarea(:,:,:) = c0
 
       !$OMP PARALLEL DO PRIVATE(iblk,i,j,ilo,ihi,jlo,jhi,this_block)
       do iblk = 1, nblocks