From's getBuildStage() and getBuildStageRange() returns null if the instruction has more than 3 arguments

[rcjsuen]

FROM node AS build abc

Although the above Dockerfile is invalid, we should still return something for getBuildStage() and getBuildStageRange() given that the third argument does exist and it follows an AS.

let dockerfileAst = require("dockerfile-ast");
dockerfile = dockerfileAst.DockerfileParser.parse("FROM node AS build abc");
console.log("Name:\t" + dockerfile.getInstructions()[0].getBuildStage());
console.log("Range:\t" + dockerfile.getInstructions()[0].getBuildStageRange());

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