[FIX] account: don't overwrite statement_line_id when reconciling transfer account

Description of the issue:

Transfer between Bank1 (550001) and Bank 2 (550002) of the same company via the intercompany bank account (580000) :

Bank 1: minus 1000 EUR, we create the following accounting entry via the reconcile widget

account 550001 credit 1000
account 580000 debit 1000

Bank 2 : Plus 1000 EUR

When we select the counterparty 580000 entry of the Bank 1 statement via the reconcile widget
than the statement_line_id of the entry from Bank 1 statement is replaced by the one of the Bank 2 statement.
Hence this account.move.line is linked to the wrong statement line.

closes odoo/odoo#42339

Signed-off-by: oco-odoo <oco-odoo@users.noreply.github.com>