[FIX] account_voucher: evaluate move line account with correct precision

This bug may have generated incorrect account_voucher_line when validating a voucher with amount less than 1. This patch will avoid reproducing the problem on new lines but not fix already existing vouchers. To do so apply the following steps:

1. apply this patch
2. do a manual reconciliation of the account.move.lines with amount less than 1 (use the manual reconciliation menu to see every line, included 0-0 lines)
3. execute the following SQL query

DELETE FROM account_voucher_line WHERE id IN (SELECT l.id FROM account_voucher_line AS l JOIN account_voucher AS v ON (v.id = l.voucher_id) JOIN account_move_line AS ml ON (l.move_line_id = ml.id) WHERE l.amount = 0 AND v.state = 'draft' AND ml.debit = ml.credit AND ml.credit = 0);

that will remove account voucher lines from draft vouchers linked to an empty move lines