Don't run upgrade_to_v320 if user.dat doesn't exist

There's nothing to upgrade if there's no legacy userbase.

exec/update.js

@@ -234,7 +234,7 @@ if(!upgraded)
 	js.exec("upgrade_to_v320.js", {});
 
 print("Checking for v3.20 user base");
-if(!file_exists(system.data_dir + 'user/user.tab')) {
+if(file_exists(system.data_dir + 'user/user.dat') && !file_exists(system.data_dir + 'user/user.tab')) {
 	var cmdline = system.exec_dir + "upgrade_to_v320";
 	print("No v3.20 user base found, running " + cmdline);
 	system.exec(cmdline);