Fix code scanning alert no. 98: Potential use after free

Co-authored-by: Copilot Autofix powered by AI <62310815+github-advanced-security[bot]@users.noreply.github.com>

tiberiandawn/terrain.cpp

@@ -537,6 +537,7 @@ void TerrainClass::AI(void)
         */
         if (IsCrumbling && Fetch_Stage() == Get_Build_Frame_Count(Class->Get_Image_Data()) - 1) {
             delete this;
+            return;
         }
     }