all: assign zero after resize in implementations of heap.Interface (ethereum#26296)

This changes the Pop method to assign the zero value before
reducing slice size. Doing so ensures the backing array does not
reference removed item values.

Author: estensen

core/txpool/list.go

@@ -45,6 +45,7 @@ func (h *nonceHeap) Pop() interface{} {
 	old := *h
 	n := len(old)
 	x := old[n-1]
+	old[n-1] = 0
 	*h = old[0 : n-1]
 	return x
 }

core/types/transaction.go

@@ -508,6 +508,7 @@ func (s *TxByPriceAndTime) Pop() interface{} {
 	old := *s
 	n := len(old)
 	x := old[n-1]
+	old[n-1] = nil
 	*s = old[0 : n-1]
 	return x
 }

p2p/util.go

@@ -70,6 +70,7 @@ func (h *expHeap) Pop() interface{} {
 	old := *h
 	n := len(old)
 	x := old[n-1]
+	old[n-1] = expItem{}
 	*h = old[0 : n-1]
 	return x
 }