message: Add null check with exception expected by net_plugin
diff: @@ -1189,7 +1189,12 @@ block_id_type controller::get_block_id_for_num( uint32_t block_num )const { try
  return blk_state->id;
  }
  - return my->blog.read_block_by_num(block_num)->id();
  + auto signed_blk = my->blog.read_block_by_num(block_num);
  +
  + EOS_ASSERT( BOOST_LIKELY( signed_blk != nullptr ), unknown_block_exception,
  + "Could not find block: ${block}", ("block", block_num) );
  +
  + return signed_blk->id();
  } FC_CAPTURE_AND_RETHROW( (block_num) ) }
  void controller::pop_block() {
  