nil pointer panic when Unmarshal with a empty file

[hanxiatu-fc]

panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xc0000005 code=0x0 addr=0x40 pc=0x595a84]

goroutine 1 [running]:
github.com/json-iterator/go.(*Iterator).ReadVal(0xc00016c000, 0x0, 0x0)
	C:/Users/f00515581/go/pkg/mod/github.com/json-iterator/go@v1.1.10/reflect.go:68 +0x2d4
github.com/json-iterator/go.(*frozenConfig).Unmarshal(0xc000058140, 0x2c270000, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0)
	C:/Users/f00515581/go/pkg/mod/github.com/json-iterator/go@v1.1.10/config.go:348 +0xd6
github.com/json-iterator/go.Unmarshal(...)
	C:/Users/f00515581/go/pkg/mod/github.com/json-iterator/go@v1.1.10/adapter.go:16
jsoniter.Fuzz(0x2c270000, 0x0, 0x0, 0x6b0)
	D:/workspace/go/fuzz_test/jsoniter/fuzz.go:19 +0x18b
go-fuzz-dep.Main(0xc000119f70, 0x1, 0x1)
	go-fuzz-dep/main.go:36 +0x1b4
main.main()
	jsoniter/go.fuzz.main/main.go:15 +0x59
exit status 2

We can reproduce the problem just by call json.Unmarshal(data, nil)

in github.com/json-iterator/go@v1.1.10/reflect.go

67		typ := reflect2.TypeOf(obj)    // when we call json.Unmarshal(anydata, nil), obj is nil, so typ is nil
68		if typ.Kind() != reflect.Ptr { // nil pointer dereference on typ

How did I find the problem?

I refer to the test cases and test corpus for std json in the go-fuzz project to test json-iterator.

see ： https://github.com/dvyukov/go-fuzz-corpus/tree/master/json