A bug when .* in the content to match

[wgplt]

The code that caused the error：

Why nil ？

This should be the right result：

Sample code：

`package main

import (
"fmt"

"github.com/dlclark/regexp2"

)

func main() {

r, err := regexp2.Compile(`(?<=1234\.\*56).*(?=890)`, regexp2.Compiled)

if err != nil {
	panic(err)
}

m, err := r.FindStringMatch(`1234.*567890`)
if err != nil {
	panic(err)
}

fmt.Println(m)

}`

[dlclark]

This was a heck of a bug!

Basically it required a specific shape to string literals within the pattern broken up with escape sequences just right so that when the pattern parser concatenated them all it re-used the underlying rune slice. The rune slice was an extract from the pattern itself, so the append calls were smashing the contents of the pattern accidentally. This meant the pattern was being parsed into a weird state and wouldn't match what you wanted.

The fix was to copy the runes from the pattern into a new slice instead of just slicing from the pattern itself. A simple fix after hours of debugging.