Refer to block as a pointer. More defensive coding.

libraries/chain/database.cpp

@@ -3532,21 +3532,24 @@ void database::update_last_irreversible_block()
       {
          while( log_head_num < dpo.last_irreversible_block_num )
          {
-            signed_block b;
+            signed_block* block_ptr;
             auto blocks = _fork_db.fetch_block_by_number( log_head_num + 1 );
 
             if( blocks.size() == 1 )
-               b = blocks[0]->data;
+               block_ptr = &( blocks[0]->data );
             else
             {
+               ilog( "Encountered a block num collision due to a fork. Walking the current fork to determine the correct block. block_num:${n}", ("n", log_head_num + 1) ); // TODO: Delete when we know this code works as intended
                auto next = _fork_db.head();
-               while( next->num > log_head_num + 1 )
+               while( next.get() != nullptr && next->num > log_head_num + 1 )
                   next = next->prev.lock();
 
-               b = next->data;
+               FC_ASSERT( next.get() != nullptr, "Current fork in the fork database does not contain the last_irreversible_block" );
+
+               block_ptr = &( next->data );
             }
 
-            _block_log.append( b );
+            _block_log.append( *block_ptr );
             log_head_num++;
          }