[BUGFIX] Syntax error in class.tx_coreupdates_t3skin.php

Fix Syntax Problem: Cannot use [] for reading

Change-Id: I9f5be90dbc9aa55475049bd8a8720cab0c321af4
Resolves: #35274
Releases: 4.7, 4.5
Reviewed-on: https://review.typo3.org/22700
Reviewed-by: Alexander Opitz
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind