Add a missing mutex lock/unlock.
Add a missing mutex lock/unlock.

Spotted by dhill.

btcsuite/btcwallet,jrick/dcrwallet,btcsuite/btcwallet,Stash-Crypto/btcwallet,jrick/btcwallet,Stash-Crypto/btcwallet,decred/dcrwallet,synrg-labs/btcwallet,decred/dcrwallet,jrick/dcrwallet,synrg-labs/btcwallet,Roasbeef/btcwallet,Roasbeef/btcwallet,jrick/btcwallet