Fix #492 - database corruption when closing

The database was attempting to pop blocks for which there was no undo
history. These changes make it impossible to pop blocks if there isn't
a fork db history.

libraries/chain/db_block.cpp

@@ -417,9 +417,10 @@ void database::pop_block()
    auto head_id = head_block_id();
    optional<signed_block> head_block = fetch_block_by_id( head_id );
    GRAPHENE_ASSERT( head_block.valid(), pop_empty_chain, "there are no blocks to pop" );
-   pop_undo();
-   _block_id_to_block.remove( head_id );
+
    _fork_db.pop_block();
+   _block_id_to_block.remove( head_id );
+   pop_undo();
 
    _popped_tx.insert( _popped_tx.begin(), head_block->transactions.begin(), head_block->transactions.end() );
 

libraries/chain/fork_database.cpp

@@ -38,7 +38,10 @@ void fork_database::reset()
 
 void fork_database::pop_block()
 {
-   if( _head ) _head = _head->prev.lock();
+   FC_ASSERT( _head, "no blocks to pop" );
+   auto prev = _head->prev.lock();
+   FC_ASSERT( prev, "poping block would leave head block null" );
+    _head = prev;
 }
 
 void     fork_database::start_block(signed_block b)