[BUGFIX] Check TBE_STYLES skins for being an array

If no skinning extension is installed, an exception is thrown which
makes it impossible to use the system. Therefore the variable must
be checked.

Resolves: #61446
Releases: 6.3
Change-Id: Iefd603199d69d0609b662f3b89a5ea6fb479e50f
Reviewed-on: http://review.typo3.org/32645


Reviewed-by: Stefan Froemken <froemken@gmail.com>
Tested-by: Stefan Froemken <froemken@gmail.com>
Reviewed-by: Xavier Perseguers <xavier@typo3.org>
Tested-by: Xavier Perseguers <xavier@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>