Change CChain::FindFork() to take ref

The internal null-guard in FindFork() was removed in favor of adding any missing guards at call sites.

src/init.cpp

@@ -2374,7 +2374,7 @@ bool StartIndexBackgroundSync(NodeContext& node)
                     LogWarning("Failed to find block manager entry for best block %s from %s, falling back to genesis for index sync",
                         summary.best_block_hash.ToString(), summary.name);
                 } else if (!index_chain.Contains(*pindex)) {
-                    pindex = index_chain.FindFork(pindex);
+                    pindex = index_chain.FindFork(*pindex);
                 }
             }
             if (!pindex) {