Change Details

`ninja && ninja check && test/functional/test_runner.py rpc_psbt`

`ninja && ninja check && test/functional/test_runner.py rpc_psbt` Check that before this patch to psbt.cpp the functional test fails because the "next" role is signer, and after this patch is has the correct "finalizer" value.

`ninja && ninja check && test/functional/test_runner.py rpc_psbt` Check that before this patch to psbt.cpp the functional test fails because the "next" role is signer, and after this patch is has the correct "finalizer" value.