Multires: Correct NULL-pointer check

There is some hard-to-reproduce scenario when top level
would have masks allocated, but without masks on the sculpt
level.

Need to check proper array before accessing its elements.
The check for top-level masks is done by the caller.

source/blender/blenkernel/intern/multires_reshape_smooth.c

@@ -127,7 +127,7 @@ static float interpolate_masks_grid(const MultiresReshapeSmoothContext *reshape_
                                     const GridCoord *grid_coord)
 {
   const MultiresReshapeContext *reshape_context = reshape_smooth_context->reshape_context;
-  if (reshape_context->grid_paint_masks == NULL) {
+  if (reshape_context->orig.grid_paint_masks == NULL) {
     return 0.0f;
   }