Improper Input Validation in CVSS v3 parsing

[pandatix]

While fuzzing this implementation, I discovered that some invalid inputs did not raise errors.
This could be categorized as CWE-20.

For instance, the following Go code does not produce any error.

package main

import (
	"fmt"

	"github.com/facebookincubator/nvdtools/cvss3"
)

func main() {
	vec, err := cvss3.VectorFromString("CVSS:3.1/AV:")

	fmt.Printf("vec: %v\n", vec)
	fmt.Printf("err: %v\n", err)
}

produces ->

vec: CVSS:3.1/
err: <nil>

You can check this input is invalid, using the official first.org calculator which does not give scores as it lacks a lot of valid info, or by looking at the specification Table 15 which shows that base metrics can't be empty.

EDIT: using version v0.1.5.