builtin/fetch: avoid aborting closed reference transaction

builtin/fetch: avoid aborting closed reference transaction

As part of the reference transaction commit phase, the transaction is
set to a closed state regardless of whether it was successful of not.
Attempting to abort a closed transaction via `ref_transaction_abort()`
results in a `BUG()`.

In c92abe71df (builtin/fetch: fix leaking transaction with `--atomic`,
2024-08-22), logic to free a transaction after the commit phase is moved
to the centralized exit path. In cases where the transaction commit
failed, this results in a closed transaction being aborted and signaling
a bug.

Free the transaction and set it to NULL when the commit fails. This
allows the exit path to correctly handle the error without attempting to
abort the transaction.

Signed-off-by: Justin Tobler <jltobler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>