eth/fetcher: remove superfluous nilness-check (#23739)

* eth/fetcher: fix nilness check https://github.com/ethereum/go-ethereum/issues/23738

* eth/fetcher: Use errors.Is. PR feedback from @holiman.

eth/fetcher/tx_fetcher.go

@@ -18,6 +18,7 @@ package fetcher
 
 import (
 	"bytes"
+	"errors"
 	"fmt"
 	mrand "math/rand"
 	"sort"
@@ -277,30 +278,28 @@ func (f *TxFetcher) Enqueue(peer string, txs []*types.Transaction, direct bool)
 	)
 	errs := f.addTxs(txs)
 	for i, err := range errs {
-		if err != nil {
 		// Track the transaction hash if the price is too low for us.
 		// Avoid re-request this transaction when we receive another
 		// announcement.
-			if err == core.ErrUnderpriced || err == core.ErrReplaceUnderpriced {
+		if errors.Is(err, core.ErrUnderpriced) || errors.Is(err, core.ErrReplaceUnderpriced) {
 			for f.underpriced.Cardinality() >= maxTxUnderpricedSetSize {
 				f.underpriced.Pop()
 			}
 			f.underpriced.Add(txs[i].Hash())
 		}
 		// Track a few interesting failure types
-			switch err {
-			case nil: // Noop, but need to handle to not count these
+		switch {
+		case err == nil: // Noop, but need to handle to not count these
 
-			case core.ErrAlreadyKnown:
+		case errors.Is(err, core.ErrAlreadyKnown):
 			duplicate++
 
-			case core.ErrUnderpriced, core.ErrReplaceUnderpriced:
+		case errors.Is(err, core.ErrUnderpriced) || errors.Is(err, core.ErrReplaceUnderpriced):
 			underpriced++
 
 		default:
 			otherreject++
 		}
-		}
 		added = append(added, txs[i].Hash())
 	}
 	if direct {