[BUGFIX] Fatal error in configuration ($BE_USER->uc)

This happens because an object is handed over as
$arr[$key].
In this case convert $arr[$key] to a real array.

Change-Id: I128de2da656a3d4782bcb9b8b2a9519aade6ce9f
Fixes: #33444
Releases: 4.6, 4.7, 6.0
Reviewed-on: http://review.typo3.org/8693
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn