ice_grid: initialize 'uvm' to 0

In subroutine 'makemask', the array 'uvm' is initialized in a loop on indices
{ilo, ihi} and {jlo, jhi}, but is then used in a loop on indices {1, nx_block} and
{1, ny_block}, causing an uninitialized value to be used (at line 1672).

Initialize the whole array to zero first, to avoid using uninitialized values.
The initialization to zero is actually commented out, following
CICE-Consortium#180, so uncomment it.

cicecore/cicedynB/infrastructure/ice_grid.F90

@@ -1635,7 +1635,7 @@ subroutine makemask
       !-----------------------------------------------------------------
 
       bm = c0
-!     uvm = c0
+      uvm = c0
 
       !$OMP PARALLEL DO PRIVATE(iblk,i,j,ilo,ihi,jlo,jhi,this_block)
       do iblk = 1, nblocks