Fix: no-extra-parens crash when code is "((let))"

[not-an-aardvark]

What is the purpose of this pull request? (put an "X" next to item)

[x] Bug fix

The PR title contains all of the information needed to reproduce the issue.

Tell us about your environment

• ESLint Version: master
• Node Version: latest
• npm Version: latest

What parser (default, Babel-ESLint, etc.) are you using?

default

Please show your full configuration:

rules:
  no-extra-parens: error

What did you do? Please include the actual source code causing the issue.

((let))

What did you expect to happen?

I expected an error to be reported by the no-extra-parens rule, because the identifier let is unnecessarily parenthesized twice.

What actually happened? Please include the actual, raw output from ESLint.

The rule crashed:

TypeError: Cannot read property 'value' of null
    at Object.isOpeningBracketToken (/home/travis/build/eslint/eslint/lib/util/ast-utils.js:329:18)
    at checkExpressionOrExportStatement (/home/travis/build/eslint/eslint/lib/rules/no-extra-parens.js:482:65)
    at ExpressionStatement (/home/travis/build/eslint/eslint/lib/rules/no-extra-parens.js:565:42)
    at listeners.(anonymous function).forEach.listener (/home/travis/build/eslint/eslint/lib/util/safe-emitter.js:45:58)
    at Array.forEach (<anonymous>)
    at Object.emit (/home/travis/build/eslint/eslint/lib/util/safe-emitter.js:45:38)
    at NodeEventGenerator.applySelector (/home/travis/build/eslint/eslint/lib/util/node-event-generator.js:251:26)
    at NodeEventGenerator.applySelectors (/home/travis/build/eslint/eslint/lib/util/node-event-generator.js:280:22)
    at NodeEventGenerator.enterNode (/home/travis/build/eslint/eslint/lib/util/node-event-generator.js:294:14)
    at CodePathAnalyzer.enterNode (/home/travis/build/eslint/eslint/lib/code-path-analysis/code-path-analyzer.js:632:23)

What changes did you make? (Give an overview)

This fixes a broken check in the no-extra-parens rule for tokens after let identifiers.

This issue was detected by the fuzzer in https://travis-ci.org/eslint/eslint/jobs/498031437.

Is there anything you'd like reviewers to focus on?

Nothing in particular

[aladdin-add]

just curious is (let) foo valid? I would have thought it will cause a parsing error.

and should we add a test (var) foo(if it is accepted).

[not-an-aardvark]

(let)\nfoocontains two ExpressionStatements, an identifier called let and an identifier called foo. ASI inserts a semicolon after (let). (let is a valid variable name in sloppy mode.)

Without parentheses, let\nfoo is a variable declaration for a variable named foo.

[mysticatea]

@aladdin-add var is a reserved word, but let is not. We can use let as a variable name.

[aladdin-add]

LGTM, thanks!