VerifyAndDecrypt panics on malformed input

[alex]

package main

import (
    "fmt"
    "time"
    "github.com/fernet/fernet-go"
)

func main() {
    a := []byte{103, 71, 70, 105, 89, 119, 61, 61}
    keys := fernet.MustDecodeKeys("AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=")
    fmt.Println(fernet.VerifyAndDecrypt(a, time.Second * 60, keys))
}

[alex]

I suggest adding a test case for an input like this (length less than the timestamp, probably also not enough space for hmac as well) to the spec tests.

[kr]

Oh snap. Thanks!

There is a test for this in https://github.com/fernet/spec/blob/f16a35/invalid.json#L10-14,
but it's clearly insufficient.