subsurface-shot-test: Don't leak replaced buffer

Destroy the buffer we've overwritten.

Signed-off-by: Daniel Stone <daniels@collabora.com>

tests/subsurface-shot-test.c

@@ -287,6 +287,7 @@ TEST(subsurface_empty_mapping)
 	struct wl_subcompositor *subco;
 	struct wp_viewporter *viewporter;
 	struct buffer *bufs[3] = { 0 };
+	struct buffer *buf_tmp;
 	struct wl_surface *surf[3] = { 0 };
 	struct wl_subsurface *sub[3] = { 0 };
 	struct wp_viewport *viewport;
@@ -385,7 +386,9 @@ TEST(subsurface_empty_mapping)
 	fail += check_screen(client, "subsurface_empty_mapping", 0, &clip, 10);
 
 	/* remap middle surface to ensure recursive mapping */
+	buf_tmp = bufs[1];
 	bufs[1] = surface_commit_color(client, surf[1], &blue, 100, 100);
+	buffer_destroy(buf_tmp);
 
 	fail += check_screen(client, "subsurface_empty_mapping", 1, &clip, 11);