wallet: Store account xpub in address buffer after upgrade
wallet: Store account xpub in address buffer after upgrade

This fixes a bug where grpc's NextAddress calls would panic due to the
account's xpub being nil.

This only affected newly created wallets immediately after the coin type
upgrade.
jrick/dcrwallet,jrick/btcwallet,decred/dcrwallet,jrick/btcwallet,jrick/dcrwallet,decred/dcrwallet