RPCs that need to use the wallet should check if it's unlocked

At some appropriate place in `issueasset` in `rpcwallet.cpp` (after `LOCK(pwallet->cs_wallet)` but before doing anything that uses the wallet), there should be a call to `EnsureWalletIsUnlocked(pwallet)`. Otherwise we crash when trying to sign if the wallet has a password and is not unlocked.

We should also check whether any other RPCs have this issue (any RPCs that need to use the wallet for signing, which I think is the only thing you need it unlocked for?)