Script API: Fix segfault in remove_detached_inventory

when minetest.remove_detached_inventory is called on script init, the environment is yet not set up,
hence m_env is still nullptr until all scripts are loaded

src/server/serverinventorymgr.cpp

@@ -157,8 +157,8 @@ bool ServerInventoryManager::removeDetachedInventory(const std::string &name)
 			m_env->getGameDef()->sendDetachedInventory(
 					nullptr, name, player->getPeerId());
 
-	} else {
-		// Notify all players about the change
+	} else if (m_env) {
+		// Notify all players about the change as soon ServerEnv exists
 		m_env->getGameDef()->sendDetachedInventory(
 				nullptr, name, PEER_ID_INEXISTENT);
 	}