Fix XSS at column 'workspace membership'.

How to test:
* choose a workspace title like "<b>test</b>"
* assign a user as member to the workspace
* select the BE-module "Admin Tools->User Admin"
* select the checkbox "Workspace membership"
* press update
* take a look at column "Workspace membership"

Change-Id: I29bb7d05a2740cc9c88eb67c224e942dfc9165dc
Fixes: #32040
Releases: 4.7, 4.6, 4.5, 4.4
Reviewed-on: http://review.typo3.org/6878
Reviewed-by: Oliver Klee
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer