[BUGFIX] Fix usage of fileadminDir

#41120 introduced a regression, as
$GLOBALS['TYPO3_CONF_VARS']['BE']['fileadminDir']
has a trailing backslash.

Fix that by removing the trailing slash and quoting
the value of the global for the case it contains
further path segments.

Fixes: #42454
Relates: #41120
Releases: 6.0, 4.7

Change-Id: I17455374ab2b9824bee9ceeaccbee7ea959b4841
Reviewed-on: http://review.typo3.org/15987
Reviewed-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn