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}, potentially 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
10c446a (Dummy and unused variables. (CICE-Consortium#180), 2018-09-22) [1], so uncomment it.

[1] CICE-Consortium#180

cicecore/cicedynB/infrastructure/ice_grid.F90

@@ -1636,7 +1636,7 @@ 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