db_lmdb: catch attempt to remove block from an empty blockchain
db_lmdb: catch attempt to remove block from an empty blockchain

It would probably have thrown when not finding a block at
height 2^64-1, but better make things clear.

eiabea/bitmonero,eiabea/bitmonero,ranok/bitmonero,eiabea/bitmonero,eiabea/bitmonero,eiabea/bitmonero,ranok/bitmonero,ranok/bitmonero,ranok/bitmonero,ranok/bitmonero,ranok/bitmonero,eiabea/bitmonero