jsoniter.Unmarshal incorrectly sets pre-initialized map[string]any to nil when input begins with letter "n" #720

Describe the bug

When unmarshaling into a pre-initialized map[string]any, if the input starts with the letter n, jsoniter.Unmarshal unexpectedly sets the map to nil.
This happens even when the input is not valid JSON (nan, none, nil, notjson...).

This behavior causes runtime panics such as:

panic: assignment to entry in nil map

because after unmarshaling, the map is unexpectedly nil.

This does not happen with Go’s standard encoding/json for invalid JSON input (standard json returns an error but does not overwrite the map).

To Reproduce

package main

import (
    "fmt"
    jsoniter "github.com/json-iterator/go"
)

func main() {
    data := map[string]any{}                  // pre-initialized map
    input := []byte("none")                   // starts with 'n' but not valid JSON

    err := jsoniter.Unmarshal(input, &data)
    fmt.Println("error:", err)
    fmt.Println("is nil:", data == nil)       // ❌ becomes nil unexpectedly

    data["a"] = 1                              // ❌ panic: assignment to entry in nil map
}

Expected behavior

If the input is invalid JSON (e.g., "none", "nan", "nil", "notjson"),
Unmarshal should return an error without modifying the existing map.

A pre-initialized map should never be overwritten with nil unless the input is literally JSON null.

This is how encoding/json behaves.

Actual behavior

For any input beginning with n, jsoniter attempts partial parsing and sets the target map to nil.

This breaks existing Go code that assumes a pre-initialized map remains non-nil after a failed unmarshal.