Since the introduction of the ScriptForMining callback, the mining
functions (setgenerate and generate) crash with an assertion failure
(due to a NULL pointer script returned) if the keypool is empty. Fix
this by giving a proper error.
An user on IRC reported an issue where `getrawchangeaddress`
keeps returning a single address when the keypool is exhausted.
In my opinion this is strange behaviour.
- Change CReserveKey to fail when running out of keys in the keypool.
- Make `getrawchangeaddress` return RPC_WALLET_KEYPOOL_RAN_OUT when
unable to create an address.
- Add a Python RPC test for checking the keypool behaviour in combination
with encrypted wallets.
