Font.php: hot fix for calculateTextWidth in case Widths key is not set

[k00ni]

Type of pull request

• Bug fix (involves code and configuration changes)

About

Hot fix for Font.php to avoid PHP warning in case the Widths-key is not set in $details.

Checklist for code / configuration changes

In case you changed the code/configuration, please read each of the following checkboxes as they contain valuable information:

• Please add at least one test case (unit test, system test, ...) to demonstrate that the change is working. If existing code was changed, your tests cover these code parts as well.
  By the way, you don't have to provide a full fledged PDF file to demonstrate a fix. Instead a unit test may be sufficient sometimes,
  please have a look at FontTest for example code.
  Code changes without any tests are likely to be rejected. If you dont know how to write tests, no problem, tell us upfront and we may add them ourselves or discuss other ways.
• Please run PHP-CS-Fixer before committing, to confirm with our coding styles. See https://github.com/smalot/pdfparser/blob/master/.php-cs-fixer.php for more information about our coding styles.

Acknowledgement

Bug reported in #619 by @gidzr.

[k00ni]

I refined the wording of my comment. Changes like these might be needed in other places too, but for now that should be sufficient.