core: free pointer from slice after popping element from price heap (#21572)

* Fix potential memory leak in price heap

* core: nil free pointer slice (alternative version)

Co-authored-by: Martin Holst Swende <martin@swende.se>

core/tx_list.go

@@ -433,6 +433,7 @@ func (h *priceHeap) Pop() interface{} {
 	old := *h
 	n := len(old)
 	x := old[n-1]
+	old[n-1] = nil
 	*h = old[0 : n-1]
 	return x
 }