blockchain: update cumulative block limit when popping a block

Avoids possible issues with accepting a tx too large to fit
in an actual block.

Reported by smooth.

src/cryptonote_core/blockchain.cpp

@@ -498,6 +498,7 @@ block Blockchain::pop_block_from_blockchain()
       }
     }
   }
+  update_next_cumulative_size_limit();
   m_tx_pool.on_blockchain_dec(m_db->height()-1, get_tail_id());
 
   return popped_block;