Fix: Never write previews for undo steps.

This may eat quite q bit of mem in the end (though nothing critical),
and icons (material ones e.g.) often ended up out of sync after undo.

source/blender/blenloader/intern/writefile.c

@@ -2057,7 +2057,8 @@ static void write_lattices(WriteData *wd, ListBase *idbase)
 
 static void write_previews(WriteData *wd, PreviewImage *prv)
 {
-	if (prv) {
+	/* Never write previews in undo steps! */
+	if (prv && !wd->current) {
 		short w = prv->w[1];
 		short h = prv->h[1];
 		unsigned int *rect = prv->rect[1];