Calling getAvailableVariables() on a Dockerfile with an invalid line number will throw an error

[rcjsuen]

We don't perform any argument checking for getAvailableVariables() so an invalid line number will cause an error to be thrown.

import { DockerfileParser } from 'dockerfile-ast';

const content =
`FROM alpine`

let dockerfile = DockerfileParser.parse(content);
let variables = dockerfile.getAvailableVariables(1);

$ node out.js
dockerfile.js:116
        return image.getAvailableVariables(currentLine);
                     ^

TypeError: Cannot read property 'getAvailableVariables' of null
    at Dockerfile.getAvailableVariables (dockerfile.js:116:22)
    at Object.<anonymous> (out.js:6:28)
    at Module._compile (module.js:643:30)
    at Object.Module._extensions..js (module.js:654:10)
    at Module.load (module.js:556:32)
    at tryModuleLoad (module.js:499:12)
    at Function.Module._load (module.js:491:3)
    at Function.Module.runMain (module.js:684:10)
    at startup (bootstrap_node.js:187:16)
    at bootstrap_node.js:608:3