nil pointer deref for xpath //, //* works

[tv42]

Either xpath.MustCompile needs to fail, or xpath.Expr.Select needs to not crash.

$ head -999 go.mod main.go
==> go.mod <==
module example.com/m

go 1.13

require (
	github.com/antchfx/htmlquery v1.1.0
	github.com/antchfx/xpath v1.1.0
	golang.org/x/net v0.0.0-20191101175033-0deb6923b6d9
)

==> main.go <==
package main

import (
	"log"
	"strings"

	"github.com/antchfx/htmlquery"
	"github.com/antchfx/xpath"
	"golang.org/x/net/html"
)

func main() {
	const doc = ``
	tree, err := html.Parse(strings.NewReader(doc))
	if err != nil {
		log.Fatalf("html parse: %v", err)
	}
	exp := xpath.MustCompile(`//`)
	_ = htmlquery.QuerySelector(tree, exp)
}
$ go run .
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x10 pc=0x6950c1]

goroutine 1 [running]:
github.com/antchfx/xpath.(*Expr).Select(...)
	github.com/antchfx/xpath@v1.1.0/xpath.go:130
github.com/antchfx/htmlquery.QuerySelector(0xc0000ca230, 0x0, 0x0)
	github.com/antchfx/htmlquery@v1.1.0/query.go:70 +0x61
main.main()
	example.com/m@/main.go:19 +0x10a
exit status 2
$