ice_dyn_vp: initialize residual vector to zero

The arrays Fx, Fy that hold the residual (Ax - b) are not initialized to
sero before being passed to 'residual_vec', and this subroutine only
loops through ice points (and not the whole of the arrays).

In debug mode, this leads to NaNs being involved in some computations
during the 'global_sum_prod' calls used to compute the norm of the
residual just after it's computed. This is only triggered in certain
tests of the 'decomp_suite'.

Initialize the residual vector (arrays Fx and Fy) to zero. This allows
the 'decomp_suite' to pass when ran with 'bfbflag = reprosum'.

cicecore/cicedynB/dynamics/ice_dyn_vp.F90

@@ -878,6 +878,8 @@ subroutine anderson_solver (icellt  , icellu , &
                             vrel     (:,:,iblk))
 
             ! Compute nonlinear residual norm (PDE residual)
+            Fx = c0
+            Fy = c0
             call matvec (nx_block             , ny_block           , &
                          icellu       (iblk)  , icellt       (iblk), &
                          indxui     (:,iblk)  , indxuj     (:,iblk), &