core: fix txpool off-by-one error (#21683)

core/tx_pool.go

@@ -817,6 +817,7 @@ func (pool *TxPool) addTxs(txs []*types.Transaction, local, sync bool) []error {
 			nilSlot++
 		}
 		errs[nilSlot] = err
+		nilSlot++
 	}
 	// Reorg the pool internals if needed and return
 	done := pool.requestPromoteExecutables(dirtyAddrs)