Instruction's getArgumentsRanges() throws TypeError if it's multiline and the final argument is a single character on the last line

[rcjsuen]

The fact that this only happens if the last argument is a single character (with no leading whitespace) makes this a rather obscure case and is probably why no one has hit this yet.

RUN x\
y

import { DockerfileParser } from 'dockerfile-ast';
const dockerfile = DockerfileParser.parse("RUN x\\\ny");
const instruction = dockerfile.getInstructions()[0];
console.log(instruction.getArgumentsRanges());

/../node_modules/vscode-languageserver-types/lib/main.js:36
            return Is.defined(candidate) && Is.number(candidate.line) && Is.number(candidate.character);
                                                                ^

TypeError: Cannot read property 'line' of null
    at Object.is (/../node_modules/vscode-languageserver-types/lib/main.js:36:65)
    at Object.create (/../node_modules/vscode-languageserver-types/lib/main.js:50:31)
    at JSONInstruction.getArgumentsRanges (/../node_modules/dockerfile-ast/lib/instruction.js:136:57)
    at Object.<anonymous> (/../out/src/main copy.js:6:25)
    at Module._compile (internal/modules/cjs/loader.js:778:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:789:10)
    at Module.load (internal/modules/cjs/loader.js:653:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:593:12)
    at Function.Module._load (internal/modules/cjs/loader.js:585:3)
    at Function.Module.runMain (internal/modules/cjs/loader.js:831:12)