IF: Update fork database fork-choice rule

libraries/chain/controller.cpp

@@ -2807,7 +2807,8 @@ struct controller_impl {
                assert(bsp->header_exts.count(if_ext_id) > 0); // in all instant_finality block headers
                const auto& if_ext = std::get<instant_finality_extension>(bsp->header_exts.lower_bound(if_ext_id)->second);
                if (if_ext.qc_claim.is_strong_qc) {
-                  auto claimed = forkdb.search_on_head_branch(if_ext.qc_claim.block_num);
+                  // claim has already been verified
+                  auto claimed = forkdb.search_branch(bsp->id(), if_ext.qc_claim.block_num);
                   if (claimed) {
                      set_if_irreversible_block_num(claimed->core.final_on_strong_qc_block_num);
                   }