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

@@ -105,7 +105,8 @@ void Plane::setBitmap(Bitmap *value)
 {
 	GUARD_DISPOSED;
 
-	value->ensureNonMega();
+	if (value)
+		value->ensureNonMega();
 
 	p->bitmap = value;
 }

src/sprite.cpp

@@ -152,7 +152,8 @@ void Sprite::setBitmap(Bitmap *bitmap)
 	if (p->bitmap == bitmap)
 		return;
 
-	bitmap->ensureNonMega();
+	if (bitmap)
+		bitmap->ensureNonMega();
 
 	p->bitmap = bitmap;
 	*p->srcRect = bitmap->rect();

src/window.cpp

@@ -715,7 +715,8 @@ void Window::setWindowskin(Bitmap *value)
 {
 	GUARD_DISPOSED;
 
-	value->ensureNonMega();
+	if (value)
+		value->ensureNonMega();
 
 	p->windowskin = value;
 }