Fix: sort-vars crash on mixed destructuring declarations

[not-an-aardvark]

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

[x] Bug fix

Tell us about your environment

• ESLint Version: master
• Node Version: 7.7.2
• npm Version: 4.1.2

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

default

Please show your full configuration:

parserOptions:
  ecmaVersion: 2015
rules:
  sort-vars: [error, { ignoreCase: true }]

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

var {a} = 1, b;

What did you expect to happen?

I expected ESLint to not crash.

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

ESLint crashed.

Cannot read property 'toLowerCase' of undefined
TypeError: Cannot read property 'toLowerCase' of undefined
    at VariableDeclaration.node.declarations.reduce (/path/to/eslint/lib/rules/sort-vars.js:49:60)
    at Array.reduce (native)
    at EventEmitter.VariableDeclaration (/path/to/eslint/lib/rules/sort-vars.js:40:35)
    at emitOne (events.js:96:13)
    at EventEmitter.emit (events.js:191:7)
    at NodeEventGenerator.enterNode (/path/to/eslint/lib/util/node-event-generator.js:39:22)
    at CodePathAnalyzer.enterNode (/path/to/eslint/lib/code-path-analysis/code-path-analyzer.js:607:23)
    at CommentEventGenerator.enterNode (/path/to/eslint/lib/util/comment-event-generator.js:98:23)
    at Controller.enter (/path/to/eslint/lib/eslint.js:928:36)
    at Controller.__execute (/path/to/eslint/node_modules/estraverse/estraverse.js:397:31)

What changes did you make? (Give an overview)

By design, sort-vars ignores destructuring assignments. However, the rule previously didn't account for the case where the first declarator of a variable declaration was a destructuring assignment -- this led to a crash. This commit updates the rule to handle this case properly.

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

Nothing in particular

[mention-bot]

@not-an-aardvark, thanks for your PR! By analyzing the history of the files in this pull request, we identified @nzakas, @vitorbal and @ilyavolodin to be potential reviewers.

[eslintbot]

LGTM