Fix slice out of bounds panic

fixes #196

gjson.go

@@ -2592,7 +2592,7 @@ func execModifier(json, path string) (pathOut, res string, ok bool) {
 // unwrap removes the '[]' or '{}' characters around json
 func unwrap(json string) string {
 	json = trim(json)
-	if len(json) >= 2 && json[0] == '[' || json[0] == '{' {
+	if len(json) >= 2 && (json[0] == '[' || json[0] == '{') {
 		json = json[1 : len(json)-1]
 	}
 	return json

gjson_test.go

@@ -2200,3 +2200,8 @@ func TestIssue195(t *testing.T) {
 		`**********{**",**,,**,**,**,**,"",**,**,**,**,**,**,**,**,**,**]`
 	Get(testJSON, testJSON)
 }
+
+func TestIssue196(t *testing.T) {
+	testJSON := `[#.@pretty.@join:{""[]""preserve"3,"][{]]]`
+	Get(testJSON, testJSON)
+}