[BUGFIX] Add missing isset() check in HTMLParser

This is a regression fix for #54091.
It has been forgotten to add an isset() check for 'fixAttrib.set'.

Releases: master
Resolves: #65239
Change-Id: Ia39ef0760dab61bdb5bbfb71e525cc96e15f3077
Reviewed-on: http://review.typo3.org/37115


Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>
Reviewed-by: Andreas Fernandez <andreas.fernandez@aspedia.de>
Tested-by: Andreas Fernandez <andreas.fernandez@aspedia.de>