EachKey using local stack for []byte and unsafe for []byte to string and getting burned

[FrankReh]

Only a bug for keys traversed by EachKey that need unescaping. Probably rare. I would never have a key with a newline in it myself ;), but the code tries to handle it so someone might get burned.

Because EachKey wants to use its own stack to store a short key in a buffer when the key needs to be unescaped, and because it does this in a loop as the data structure levels are traversed, an earlier cached key's value can be modified. Here's a small example of a single path that works if there are no newlines in the strings but fails as is because of this bug. Fix for me is to change the array size of the special stack variable to 0 in EachKey, effectively disabling use of the stack, so a []byte buffer is allocated each time there is an unescaped key in a path.

package main

import (
	"fmt"
	"github.com/buger/jsonparser"
)

func ExampleEachKeyBrokenWithBackingStore() {
	raw := `{"a\n":{"b\n":99}}`
	paths := [][]string{
		[]string{"a\n", "b\n"},
	}
	jsonparser.EachKey([]byte(raw),
		func(idx int, value []byte, dataType jsonparser.ValueType, err error) {
			fmt.Println("matches", idx, string(value))
		}, paths...)

	// Output:
	// matches 0 99
}