Change CChain::Next() to take reference

To minimize chance of erroneous nullptr dereference, `CChain::Next()`
is changed to take a reference instead of a pointer.
Call sites have been adapted. Notably, NextSyncBlock() now checks
the FindFork() result before calling into Next(), because
the fork lookup may return null.

src/rest.cpp

@@ -230,7 +230,7 @@ static bool rest_headers(const std::any& context,
             if (headers.size() == *parsed_count) {
                 break;
             }
-            pindex = active_chain.Next(pindex);
+            pindex = active_chain.Next(*pindex);
         }
     }
 
@@ -556,7 +556,7 @@ static bool rest_filter_header(const std::any& context, HTTPRequest* req, const
             headers.push_back(pindex);
             if (headers.size() == *parsed_count)
                 break;
-            pindex = active_chain.Next(pindex);
+            pindex = active_chain.Next(*pindex);
         }
     }