Battle UFO: fix spawn ordering logic

If the mapthings went in reverse order, so mapthing #1 had
a higher ID than mapthing #2, the spawn ordering logic
would get stuck because it wanted mapthing #1 to have
a lower ID than mapthing #2.

The root of the bug is that the previously used spawners
ID was not totally ignored. Now it is.

src/objects/battle-ufo.cpp

@@ -82,7 +82,7 @@ public:
 		auto min = [&](auto cmp) { return std::min_element(list_.begin(), list_.end(), cmp); };
 
 		return *(it != list_.end()
-			? min([order](T a, T b) { return order < a->id() && a->id() < b->id(); })
+			? min([order](T a, T b) { return order < a->id() && (b->id() <= order || a->id() < b->id()); })
 			: min([](T a, T b) { return a->id() < b->id(); }));
 	}