BlockchainLMDB: align new alt block entries in the DB

Author: jeffro256

src/blockchain_db/lmdb/db_lmdb.cpp

@@ -4488,7 +4488,8 @@ void BlockchainLMDB::add_alt_block(const crypto::hash &blkid, const cryptonote::
 
   MDB_val k = {sizeof(blkid), (void *)&blkid};
   const size_t val_size = sizeof(alt_block_data_t) + blob.size();
-  std::unique_ptr<char[]> val(new char[val_size]);
+  std::unique_ptr<char[]> val(new (std::align_val_t{alignof(alt_block_data_t)}) char[val_size]);
+  assert(reinterpret_cast<std::uintptr_t>(val.get()) % alignof(alt_block_data_t) == 0); // aligned to `alt_block_data_t`
   memcpy(val.get(), &data, sizeof(alt_block_data_t));
   memcpy(val.get() + sizeof(alt_block_data_t), blob.data(), blob.size());
   MDB_val v = {val_size, (void *)val.get()};