bugfix: Use WHDLoad custom1-5 values if they are not zero (fixes #1328)

Since we always set the default value to zero, this should be safe to use.
Apparently some games do not have some custom fields set, but still use them if specified (e.g. BattleIsle)

src/osdep/amiberry_whdbooter.cpp

@@ -1162,7 +1162,7 @@ void create_startup_sequence()
 	// CUSTOM options
 	for (int i = 1; i <= 5; ++i) {
 		auto& custom = whdload_prefs.selected_slave.get_custom(i);
-		if (custom.type != none && custom.value != 0) {
+		if (custom.value != 0) {
 			whd_bootscript << " CUSTOM" << i << "=" << custom.value;
 		}
 	}