fix iterator traversal empty object panic

[linuxsong]

Traversal a empty object with HasNext() will cause a panic.
Reproduce bug, in ast/iterator_test.go

str, _ := getTestIteratorSample(0)
root, err := NewParser(str).Parse()
if err != 0 {
    t.Fatal(err)
}
mi, _ := root.Get("object").Properties()
if mi.HasNext() {
    t.Fatalf("should not have next")
}





panic: runtime error: invalid memory address or nil pointer dereference [recovered]
    panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0x143bf91]

goroutine 8 [running]:
testing.tRunner.func1.2({0x14af1a0, 0x189a320})
    /usr/local/Cellar/go/1.19.1/libexec/src/testing/testing.go:1396 +0x24e
testing.tRunner.func1()
    /usr/local/Cellar/go/1.19.1/libexec/src/testing/testing.go:1399 +0x39f
panic({0x14af1a0, 0x189a320})
    /usr/local/Cellar/go/1.19.1/libexec/src/runtime/panic.go:884 +0x212
github.com/bytedance/sonic/ast.(*Iterator).HasNext(0xc000004cb8?)
    /Users/xks/gitproject/sonic/ast/iterator.go:70 +0x51
github.com/bytedance/sonic/ast.TestIterator(0xc0002ae680)
    /Users/xks/gitproject/sonic/ast/iterator_test.go:205 +0x937
testing.tRunner(0xc0002ae680, 0x15506c0)
    /usr/local/Cellar/go/1.19.1/libexec/src/testing/testing.go:1446 +0x10b
created by testing.(*T).Run
    /usr/local/Cellar/go/1.19.1/libexec/src/testing/testing.go:1493 +0x35f

[AsterDY]

LGTM