Validation crashes if the last instruction spans multiple lines and the final argument is one character long with no leading whitespace

[rcjsuen]

The root cause is due to rcjsuen/dockerfile-ast#66.

RUN x\
y

$ docker run -v `pwd`/Dockerfile:/Dockerfile rcjsuen/dockerfile-utils lint /Dockerfile
/dockerfile-utils/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 (/dockerfile-utils/node_modules/vscode-languageserver-types/lib/main.js:36:65)
    at Object.create (/dockerfile-utils/node_modules/vscode-languageserver-types/lib/main.js:50:31)
    at JSONInstruction.getArgumentsRanges (/dockerfile-utils/node_modules/dockerfile-ast/lib/instruction.js:136:57)
    at JSONInstruction.getArgumentsContent (/dockerfile-utils/node_modules/dockerfile-ast/lib/instruction.js:152:27)
    at Validator.checkJSONQuotes (/dockerfile-utils/lib/dockerValidator.js:1032:39)
    at Validator.validateInstruction (/dockerfile-utils/lib/dockerValidator.js:368:26)
    at Validator.validate (/dockerfile-utils/lib/dockerValidator.js:298:18)
    at Object.validate (/dockerfile-utils/lib/main.js:135:22)
    at validate (/dockerfile-utils/bin/dockerfile-utils:161:39)
    at Object.<anonymous> (/dockerfile-utils/bin/dockerfile-utils:35:5)