Fix #16227: Assertion hit in fix_duplicated_banners()

Refactor error, introduced in 63a2e9d716#

src/openrct2/world/Banner.cpp

@@ -285,7 +285,7 @@ void fix_duplicated_banners()
                         log_error("Failed to create new banner.");
                         continue;
                     }
-                    Guard::Assert(!activeBanners[index]);
+                    Guard::Assert(!activeBanners[newBanner->id.ToUnderlying()]);
 
                     // Copy over the original banner, but update the location
                     const auto* oldBanner = GetBanner(bannerIndex);