Fixed segfault with checkpoints loading

Bounds checking on blockchain_storage' m_blocks.size() when validating
against checkpoints.  Also moved initial json & DNS checkpoints load to
after blockchain init.

src/cryptonote_core/blockchain_storage.cpp

@@ -1793,6 +1793,12 @@ bool blockchain_storage::update_checkpoints(const std::string& file_path)
 
   for (const auto& pt : points)
   {
+    // if the checkpoint is for a block we don't have yet, move on
+    if (pt.first >= m_blocks.size())
+    {
+      continue;
+    }
+
     if (!m_checkpoints.check_block(pt.first, get_block_hash(m_blocks[pt.first].bl)))
     {
       // if we're enforcing dns checkpoints, roll back to a couple of blocks before the checkpoint

src/cryptonote_core/cryptonote_core.cpp

@@ -150,6 +150,10 @@ namespace cryptonote
     r = m_blockchain_storage.init(m_config_folder, testnet);
     CHECK_AND_ASSERT_MES(r, false, "Failed to initialize blockchain storage");
 
+    // load json & DNS checkpoints, and verify them
+    // with respect to what blocks we already have
+    update_checkpoints();
+
     r = m_miner.init(vm, testnet);
     CHECK_AND_ASSERT_MES(r, false, "Failed to initialize blockchain storage");
 

src/daemon/daemon.cpp

@@ -221,8 +221,6 @@ int main(int argc, char* argv[])
   } else {
     ccore.set_checkpoints(std::move(checkpoints));
     ccore.set_checkpoints_file_path(checkpoint_json_hashfile_fullpath.string());
-    res = ccore.update_checkpoints();
-    CHECK_AND_ASSERT_MES(res, 1, "Failed to load initial checkpoints");
   }
 
   cryptonote::t_cryptonote_protocol_handler<cryptonote::core> cprotocol(ccore, NULL);