Panic seen when parsing invalid 'exp' claim attribute

[pkmachani]

in version 4.0.0 of library, I noticed in my tests there was panic seen when parsing the 'exp' claim attribute, it appears from code the validation of parsed value did not check for error and dereferenced causing it to panic.

claim values used for testing
{ "iss": "87190e4d-db7a", "sub": "714a4cfc-9ba7", "aud": "5GCORE", "scope": "somescope", "exp": "not/an/actual/date" }

algorithm: RS256

File time.go

// UnmarshalJSON implements the json package's Unmarshaler interface
func (t *Time) UnmarshalJSON(data []byte) error {
	var value json.Number
	err := json.Unmarshal(data, &value)
	if err != nil {
		return err
	}
	v, err := ParseTime(value)

	*t = *v <== dereference causing panic

	return err
}

Panic:

        /usr/lib/go/src/runtime/panic.go:679 +0x1b2  github.com/dgrijalva/jwt-go/v4.(*Time).UnmarshalJSON(0xc0001a5700, 0xc000160468, 0x14, 0x15, 0x7f6c4451afd8, 0xc0001a5700)
        /root/go/pkg/mod/github.com/dgrijalva/jwt-go/v4@v4.0.0-preview1/time.go:70 +0xc4 encoding/json.(*decodeState).literalStore(0xc0002efb80, 0xc000160468, 0x14, 0x15, 0xc07c00, 0xc0001943a8, 0x196, 0x0, 0xbd, 0xc0002efba8)
        /usr/lib/go/src/encoding/json/decode.go:918 +0x2b7b