Fix nullpointer deref

src/bitmap.cpp

@@ -645,6 +645,9 @@ SDL_Surface *Bitmap::megaSurface()
 
 void Bitmap::ensureNonMega()
 {
+	if (isDisposed())
+		return;
+
 	GUARD_MEGA;
 }
 

src/plane.cpp

@@ -105,6 +105,7 @@ void Plane::setBitmap(Bitmap *value)
 {
 	GUARD_DISPOSED;
 
+	if (value)
 		value->ensureNonMega();
 
 	p->bitmap = value;

src/sprite.cpp

@@ -152,6 +152,7 @@ void Sprite::setBitmap(Bitmap *bitmap)
 	if (p->bitmap == bitmap)
 		return;
 
+	if (bitmap)
 		bitmap->ensureNonMega();
 
 	p->bitmap = bitmap;

src/window.cpp

@@ -715,6 +715,7 @@ void Window::setWindowskin(Bitmap *value)
 {
 	GUARD_DISPOSED;
 
+	if (value)
 		value->ensureNonMega();
 
 	p->windowskin = value;