nil point dereference in cartridge loading fixed

this would be happen when supercharger failed to load due to a missing BIOS file

gui/crt/preferences.go

@@ -83,7 +83,7 @@ const (
 	blackLevel         = 0.06
 	pixelPerfectFade   = 0.4
 	syncSpeedScanlines = 65
-	syncPowerOn        = false
+	syncPowerOn        = true
 )
 
 // NewPreferences is the preferred method of initialisation for the Preferences type.

hardware/memory/cartridge/cartridge.go

@@ -177,6 +177,12 @@ func (cart *Cartridge) Attach(cartload cartridgeloader.Loader) error {
 
 	// log result of Attach() on function return
 	defer func() {
+		// we might have arrived here as a result of an error so we should
+		// check cart.mapper before trying to access it
+		if cart.mapper == nil {
+			return
+		}
+
 		if _, ok := cart.mapper.(*ejected); !ok {
 			logger.Logf("cartridge", "inserted %s", cart.mapper.ID())
 		}