[BUGFIX] Fix path check for custom RTE styles

Fix for #29899 introduced a check if the file holding
custom RTE CSS exists. However that path used was
wrong.

Change-Id: I1b4f23bb6bc29add925a956636fe3b8dd01f6b86
Resolves: #32625
Releases: 4.5, 4.6, 4.7
Reviewed-on: http://review.typo3.org/7314
Reviewed-by: Stanislas Rolland
Tested-by: Stanislas Rolland
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind
Reviewed-by: Julian Hofmann
Tested-by: Julian Hofmann
Reviewed-by: Steffen Ritter
Tested-by: Steffen Ritter