blockchain_export: fix buffer overflow in exporter

Coverity 182550

src/blockchain_utilities/blocksdat_file.cpp

@@ -106,7 +106,7 @@ void BlocksdatFile::write_block(const crypto::hash& block_hash)
   {
     crypto::hash hash;
     crypto::cn_fast_hash(m_hashes.data(), HASH_OF_HASHES_STEP * sizeof(crypto::hash), hash);
-    memmove(m_hashes.data(), m_hashes.data() + HASH_OF_HASHES_STEP * sizeof(crypto::hash), (m_hashes.size() - HASH_OF_HASHES_STEP) * sizeof(crypto::hash));
+    memmove(m_hashes.data(), m_hashes.data() + HASH_OF_HASHES_STEP, (m_hashes.size() - HASH_OF_HASHES_STEP) * sizeof(crypto::hash));
     m_hashes.resize(m_hashes.size() - HASH_OF_HASHES_STEP);
     const std::string data(hash.data, sizeof(hash));
     *m_raw_data_file << data;