Fix crash caused by Lua error during startup (#7473)

src/emerge.cpp

@@ -146,6 +146,9 @@ EmergeManager::~EmergeManager()
 		}
 
 		delete thread;
+
+		// Mapgen init might not be finished if there is an error during startup.
+		if (m_mapgens.size() > i)
 			delete m_mapgens[i];
 	}