[backport#13546] wallet: Fix use of uninitialized value bnb_used in CWallet::CreateTransaction(...)

Summary:
wallet: Avoid potential use of unitialized value bnb_used in CWallet::CreateTransaction(...) (practicalswift)

Pull request description:

Avoid use of uninitialized value `bnb_used` in `CWallet::CreateTransaction(...)`.

https://github.com/bitcoin/bitcoin/pull/13546/commits/a23a7f60aa07de52d23ff1f2034fc43926ec3520

Backport of Core PR13546

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7060