fix for 2 crashes

Author: Pentarctagon

src/sdl/surface.cpp

@@ -96,6 +96,9 @@ surface& surface::operator=(surface&& s) noexcept
 
 surface surface::clone() const
 {
+	if(!surface_) {
+        return {};
+    }
 	// Use SDL_ConvertSurfaceFormat to make a copy
 	return surface(SDL_ConvertSurface(surface_, SDL_PIXELFORMAT_ARGB8888));
 }