Remove doctype node after node elements have been analyzed

[ohader]

This is really a strange scenario - however in the end it occasionally caused segmentation faults...

vendor/bin/phpunit --no-coverage --filter=/doctypeAndEntityAreRemoved/
PHPUnit 6.5.14 by Sebastian Bergmann and contributors.

[1]    63007 segmentation fault  vendor/bin/phpunit --no-coverage --filter=/doctypeAndEntityAreRemoved/

PR #53 adds corresponding entityTest.svg (used in test-case doctypeAndEntityAreRemoved) which defined a XML entity using <!DOCTYPE fortiguard [ <!ENTITY lab "cool, text as an image">]>.

It turned out that the sequence of removing doctype, followed by \DOMXPath on the document causes a segmentation fault (at least on PHP 7.2 and 7.4, using libxml2 2.9.10 (always) and 2.9.12 (occasionally)). This is the call stack:

• https://github.com/darylldoyle/svg-sanitizer/blob/0.14.0/src/Sanitizer.php#L211
• https://github.com/darylldoyle/svg-sanitizer/blob/0.14.0/src/Sanitizer.php#L216

• svg-sanitizer/src/ElementReference/Resolver.php

  Line 87 in beff895

  $elements = $this->xPath->query('//*[@id]');

• the process ends with segmentation fault when invoking that XPath query