blockchain: sanity check number of precomputed hash of hash blocks

Coverity 142951

src/cryptonote_core/blockchain.cpp

@@ -4343,8 +4343,13 @@ void Blockchain::load_compiled_in_block_hashes()
     {
       const unsigned char *p = get_blocks_dat_start(m_testnet);
       const uint32_t nblocks = *p | ((*(p+1))<<8) | ((*(p+2))<<16) | ((*(p+3))<<24);
+      if (nblocks > (std::numeric_limits<uint32_t>::max() - 4) / sizeof(hash))
+      {
+        MERROR("Block hash data is too large");
+        return;
+      }
       const size_t size_needed = 4 + nblocks * sizeof(crypto::hash);
-      if(nblocks > 0 && nblocks * HASH_OF_HASHES_STEP > m_db->height() && get_blocks_dat_size(m_testnet) >= size_needed)
+      if(nblocks > 0 && nblocks > (m_db->height() + HASH_OF_HASHES_STEP - 1) / HASH_OF_HASHES_STEP && get_blocks_dat_size(m_testnet) >= size_needed)
       {
         p += sizeof(uint32_t);
         m_blocks_hash_of_hashes.reserve(nblocks);