[BUGFIX] Escape title tag of image links

This patch adds a missing call to htmlspecialchars() when
the title tag of image links is initialized.

Resolves: #50760
Releases: 6.2, 6.1, 6.0, 4.7, 4.5
Change-Id: If41f33d9621f7790c0ff0de4aebcd7cdcb59707f
Reviewed-on: https://review.typo3.org/23348
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind