blockchain: pop forked blocks only when DB is not read-only

src/cryptonote_core/blockchain.cpp

@@ -409,7 +409,7 @@ bool Blockchain::init(BlockchainDB* db, const network_type nettype, bool offline
   m_db->block_txn_stop();
 
   uint64_t num_popped_blocks = 0;
-  while (true)
+  while (!m_db->is_read_only())
   {
     const uint64_t top_height = m_db->height() - 1;
     const crypto::hash top_id = m_db->top_block_hash();