Crash comparing multi-line strings

[novemberborn]

Originally reported in https://github.com/avajs/ava/issues/1935, comparing these two strings crashes:

`<table>
    <tr>
      <td>undefined</td>
      <td>3</td>
    </tr>
</table>
`

`<table>
    <tr>
      <td>3</td>
      <td>4</td>
    </tr>
</table>`

 Error thrown in test:

  TypeError {
    message: 'Cannot read property \'substring\' of undefined',
  }

  diff_main (node_modules/fast-diff/diff.js:63:17)
  diffLine (node_modules/concordance/lib/primitiveValues/string.js:65:19)
  StringValue.diffDeep (node_modules/concordance/lib/primitiveValues/string.js:271:24)
  Object.diffDescriptors (node_modules/concordance/lib/diff.js:307:17)
  Test.is (test/222.test.js:17:5)

Note the first string has a newline after </table>. The error occurs on this line:

https://github.com/concordancejs/concordance/blob/262782ac7f9de37d05c0f5460884ddc4b92e0c85/lib/primitiveValues/string.js#L271

Presumably expectedLines[expectedIndex] is the undefined one. This should have been picked up as "actual-is-extraneous". Why it isn't requires investigation.