For a split transaction, either fw_core_handle_response() or split_transaction_timeout_callback() may manipulate the same transaction entry in the pending transaction list. The return value of timer_delete() is used to determine which function handles the entry. If the transaction is a split transaction and timer_delete() returns zero, the timeout callback may already be running. In this case, the response handler must leave the entry in the list for the timeout callback to remove.
A previous code refactoring broke this behavior, introducing a race condition in which both functions could process the same transaction. This could lead to a use-after-free on the transaction initiator side.
Fix the regression by ensuring that the response handler does not process the transaction when timer_delete() returns zero. The timeout callback is responsible for removing the entry from the pending transaction list.