radeonsi: silence Coverity warning

Coverity's analysis is too weak to understand that
r600_init_flushed_depth(_, _, NULL) only returns true when
flushed_depth_texture was assigned a non-NULL value.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/gallium/drivers/radeonsi/si_blit.c

@@ -342,6 +342,8 @@ si_flush_depth_texture(struct si_context *sctx,
 		unsigned fully_copied_levels;
 		unsigned levels = 0;
 
+		assert(tex->flushed_depth_texture);
+
 		if (util_format_is_depth_and_stencil(dst->resource.b.b.format))
 			copy_planes = PIPE_MASK_Z | PIPE_MASK_S;
 

src/gallium/drivers/radeonsi/si_state.c

@@ -3001,6 +3001,8 @@ si_create_sampler_view_custom(struct pipe_context *ctx,
 			return NULL;
 		}
 
+		assert(tmp->flushed_depth_texture);
+
 		/* Override format for the case where the flushed texture
 		 * contains only Z or only S.
 		 */