Go: encoding/json: panics on NaN map keys

[dsnet]

Using go1.16

Reproduction:
```go
type F64 float64

func (f F64) MarshalText() ([]byte, error) {
return []byte(fmt.Sprint(f)), nil
}

func main() {
m := map[F64]string{F64(math.NaN()): "NaN"}
json.Marshal(m)
}
````

Panics with panic: reflect: call of reflect.Value.Type on zero Value

[gopherbot]

Change https://golang.org/cl/278632 mentions this issue: encoding/json: fix invalid kind reflect type

[bcmills]

See previously #14427, #24075, #20660. (NaN keys are trouble!)

[griesemer]

More precisely: NaN ~keys~ are trouble.

[mengzhuo]

More precisely: NaN ~keys~ are trouble.

Float as key is unsuppored, however this bug is about marhsal maps with interface MarshalText :)