Image.cpp: fix warning

Source/Urho3D/Resource/Image.cpp

@@ -1353,8 +1353,9 @@ bool Image::SaveDDS(const String& fileName) const
     ddsd.ddpfPixelFormat_.dwRGBAlphaBitMask_ = 0xff000000;
 
     outFile.Write(&ddsd, sizeof(ddsd));
-    for (unsigned i = 0; i < levels.Size(); ++i)
-        outFile.Write(levels[i]->GetData(), levels[i]->GetWidth() * levels[i]->GetHeight() * 4);
+
+    for (const Image* level : levels)
+        outFile.Write(level->GetData(), level->GetWidth() * level->GetHeight() * 4);
 
     return true;
 }