wallet/wallet: assume not current if best height is 0
wallet/wallet: assume not current if best height is 0

This is done to avoid the birthday rescan to fail if the chain backend
reports a bestheight of 0.

Earlier it could happen that we attempted to sync to the birthday, but
since only the genesis block was available, which would be rejected as
birthday block because of the timestamp, it would fail to find a block
and the sync would fail.

btcsuite/btcwallet,btcsuite/btcwallet