A pack of nits for rpcwallet.cpp

Summary: Mostly braces, but a few other minor changes as well.

Test Plan:

make check
../qa/pull-tester/rpc-tests.py

Reviewers: freetrader, CCulianu, #bitcoin_abc

Reviewed By: CCulianu, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D454