Get() key search can bleed through levels of JSON hierarchy #5

I want to first thank you, @buger, for your work on this library. Looking up a few JSON key paths in large JSON blobs is a significant bottleneck in a project I'm working on, and your library could give us a big speedup without changing our data format.

Unfortunately, I've discovered an issue in Get(): when searching for a key, Get() may locate that key outside the current JSON object. Here is an example test case that breaks (written using check.v1):

package jsonparser_test

import (
    "github.com/buger-jsonparser"
    . "gopkg.in/check.v1"
    "testing"
)

func (s *JsonParserTests) TestJsonParserSearchBleed(c *C) {
    killer := []byte(`{
      "parentkey": {
        "childkey": {
          "grandchildkey": 123
        },
        "otherchildkey": 123
      }
    }`)

    var jtype int

    _, jtype, _, _ = jsonparser.Get(killer, "childkey")
    c.Assert(jtype, Equals, jsonparser.NotExist) // fails, returns data parentkey.childkey

    _, jtype, _, _ = jsonparser.Get(killer, "parentkey", "childkey", "otherchildkey")
    c.Assert(jtype, Equals, jsonparser.NotExist) // fails, returns data parentkey.otherchildkey
}

// Boilerplate
func Test(t *testing.T) { TestingT(t) }
type JsonParserTests struct{}
var _ = Suite(&JsonParserTests{})

The issue is that Get() uses bytes.Index() to find the next key it's looking for, but only validates it by checking that it is surrounded by double quotes and followed by a colon. In particular, it does not check whether it has crossed an unmatched sequence of braces, which would indicate transitioning into another JSON object level.

I don't have a great suggestion as to how to fix this, sadly. Best of luck.