eth/tracers: fix release issue

eth/tracers/api.go

@@ -381,12 +381,17 @@ func (api *API) traceChain(start, end *types.Block, config *TraceConfig, closed
 				logged = time.Now()
 				log.Info("Tracing chain segment", "start", start.NumberU64(), "end", end.NumberU64(), "current", number, "transactions", traced, "elapsed", time.Since(begin))
 			}
-			// Retrieve the parent state to trace on top
+			// Retrieve the parent block and target block for tracing.
 			block, err := api.blockByNumber(ctx, rpc.BlockNumber(number))
 			if err != nil {
 				failed = err
 				break
 			}
+			next, err := api.blockByNumber(ctx, rpc.BlockNumber(number+1))
+			if err != nil {
+				failed = err
+				break
+			}
 			// Prepare the statedb for tracing. Don't use the live database for
 			// tracing to avoid persisting state junks into the database. Switch
 			// over to `preferDisk` mode only if the memory usage exceeds the
@@ -402,11 +407,6 @@ func (api *API) traceChain(start, end *types.Block, config *TraceConfig, closed
 				failed = err
 				break
 			}
-			next, err := api.blockByNumber(ctx, rpc.BlockNumber(number+1))
-			if err != nil {
-				failed = err
-				break
-			}
 			// Clean out any pending derefs. Note this step must be done after
 			// constructing tracing state, because the tracing state of block
 			// next depends on the parent state and construction may fail if