message: allow popping back to null, this is the initial state of the fork database and if you have overly aggressive test cases that do bad things on the very first block of the chain, we can and should pop back to null
diff: @@ -20,7 +20,6 @@ void fork_database::pop_block()
  {
  FC_ASSERT( _head, "no blocks to pop" );
  auto prev = _head->prev.lock();
  - FC_ASSERT( prev, "poping block would leave head block null" );
  _head = prev;
  }
  